This is really rough, and it only applies (if at all) to sensors that use similar technology (e.g. the ubiquitous Sony sensors that show up in everything from iPhones to the IQ4 150). There's probably a similar guideline for other sensor technology trees (Canon), but the points along them may very well be different.
To me, having looked at a lot of prints from a lot of different cameras over the years, there are three scales that matter - sensor size, pixel count and technology level. A "one class" jump in any of them is barely visible on the print (assuming that you aren't already beyond the resolution capability of the printer, in which case the jump
may be irrelevant - factors beyond detail resolution, such as dynamic range, can render a better sensor relevant even when the resolution of the lesser sensor seems to be enough). Two to three jumps (which can be from a combination of categories) are probably enough to open up a new print size.
The rough scales I propose here are about overall image quality at low ISO - not pure resolution. I'm a landscape photographer who likes to print big, lives for shadow detail and rarely raises ISO, so this is focused on the type of photography I do - a sports photographer would probably want something quite different (I have a friend who shoots a lot of action with his E-M1 mk II, and would be terribly frustrated by my Z7 even though it has a sensor 6 classes better on this scale).
This also assumes equivalent lenses and stabilization - An X-H1 with the 16-55 f2.8 is going to outperform a D3500 with an unstabilized kit 18-55 f3.5-5.6 (by a lot), even though they pretty much share a sensor. I'm not accounting for computational photography here - figuring that most things you can do with one camera, you can do with another (even if one has it built in and the other needs a computer).
I'm not accounting for the fact that you can have less visible resolution in a larger print, because I like BJL's concept that a very high-resolution big print is likely to bring the viewer in close, assuming the print is hanging where that's possible.
There are about 15 classes from the lowest-end camera likely to still be in use (a 4 year old iPhone or a D70 generation DSLR) to the absolute state of the art (a Phase One IQ4 150). There are also probably 4 or 5 jumps (or more???) accounted for by lens quality - but the quality of the lens isn't related to the quality of the sensor. Nobody's going to use a Lensbaby on a Phase (unless they're after a very specific effect), but there have been plenty of images made with combinations like a Nikkor 600mm f4 on a D70 or a Canon 70-200 f2.8 on an EOS-10D. There is also the issue of modern APS-C DSLRs with very good to excellent sensors but kit lenses built to a very low price point.
Sensor size:
1/2.5" (phone)<2/3" (compact)<1" (premium compact)<Micro 4/3<APS-C<24x36mm (full-frame)<33x44mm (small MF)<645ish (Phase One)
Resolution:
6 MP<8 MP<12MP<16 MP<24 MP<30-36 MP<43-50 MP<75 MP (no easy examples)<100 MP<150 MP
Generation:
On average, every 2-3 years brings a sensor generation, often accompanied by a new buzzword from Sony. The most recent important one is "BSI".
A few examples from cameras I've either owned or know files from well - just to see if they pass the sniff test... Since I own a Z7, I write Z7 when what I mean is the clumsy construction Z7/D850/A7rII/A7rIII/S1R)
Fuji X-T2 to Nikon Z7 (my two most recent cameras, with thousands of images on each) - separated by 4 classes - one in sensor size, one in generation and two in resolution. This should open up a new print size and then a little more - and it does. The X-T2 and X-H1 print 24x36" acceptably, while the Z7 is really superb at 24x36", significantly better than a Fuji at 24x36" when the Z7 is printing 30x45", and my guess is that it'll be close to as good at 40x60" as the Fujis are at 24x36".
Olympus E-M1 mk II to Fuji X-T2 (haven't owned the Olympus, but have helped a friend print many images from his) - separated by a bit more than two classes - one in sensor size, plus partial classes in resolution and sensor generation. I'm about as comfortable with the Olympus at 18x24" as I am with the Fuji at 24x36", which makes sense. The Olympus is excellent at 12x16", acceptable at 18x24".
Going the other way:
Fuji GFX 50 (either one) to Z7 - one class up in sensor size, but a reverse jump in generation. They should be very close, and they are. I examined big prints from both very carefully before buying the Nikon, and the differences were barely perceptible at best.
Z7 or GFX 50 Fuji GFX 100 - three class difference (two in resolution and one in either generation (older 50 MP MF sensor) or sensor size (Z7)). I haven't seen a print from a GFX 100, but it should make a 40x60" as good as a Z7 print at 30x45", maybe a little better than that. It probably opens up the next print size (60x90"
).
GFX 100 to Phase One IQ4 150 - one class (resolution) - this is really probably just firming up the 60x90" print size...
And jumping WAY down...
Fuji X-T2 to iPhone 6S+ is 8 classes down, consisting of three jumps from 24 MP to 8 MP plus four sensor sizes plus lens quality. The sensors are about the same generation - a newer iPhone would be a generation or even two newer, but the 6S+ is about the same as the Fuji. Lose 3 print sizes or so - from acceptable 24x36" on the Fuji, I wouldn't expect the iPhone to do 16x24" or 12x18", but it might make an acceptable 8x12" (or a good 5x7"). I have struggled to get even that out of phones, largely due to dynamic range. Can a good modern phone (12 MP with a sensor no more than a year or so old) produce a really good 8x12" print (or any real attempt at 12x18")?
I have gotten good 8x12" prints out of Nikon D70 generation DSLRs with a decent lens (after dealing with the color, which is, to put it charitably, odd by modern standards). Those should come out about the same as an iPhone 6S+ on this scale (sensor four sizes larger, but one resolution lower and something like three or four generations older).
A representative camera or two per class (many of the older ones are from memory, and may not be fully accurate - the last few are conjectural - I've never used a Phase extensively, and I've only examined prints from them at trade shows).
1.) Nikon D70, iPhone 6S+
2.) iPhone 8
3.) Nikon P1000 superzoom compact, iPhone XS
4.)Nikon D200
5.) Nikon D2x, Sony RX100
6.) Olympus E-M5 (original)
7.)Olympus E-M1 mk II
8.) Sony NEX-7
9.) Fuji X-T2
10.) Nikon D750
11.)Sony A7 mk III
12.) Pentax K1 (and the discontinued members of the 36 MP FF club)
13.)Nikon Z7, D850, Sony A7r mk III, Panasonic S1R, Fuji GFX 50 - the state of the art without going to medium format, plus the venerable 50 MP medium format sensor
14.) Doesn't exist - probably ~75 MP full frame or 50 MP small medium format with a modern sensor.
15.) Obscure - perhaps the Phase One IQ2 80 (but that loses quality
quickly above base ISO)
16.) Fuji GFX 100, Phase One IQ3 100
17.) Phase One IQ4 150
Film and print equivalents - when I say "film", I'm thinking about ISO 100 color transparency film of a reasonable grade (Provia, for example) in a camera and lens combination that's not holding it back, scanned on an appropriate scanner for the format. Yes, you can get far more resolution out of Tech Pan in Microdol, and Kodak Gold 800 in a 1993 Pentax super-duper zoom scanned on an early digital minilab will be much lower. The dynamic range of all but the oldest or smallest digital sensors will far exceed Provia, of course..
Roughly (for high detail landscape, and assuming you can get the color and tonality the way you want it - this may not be trivial with some older cameras)...
35mm film is somewhere around Class 2, and a Class 2 sensor makes a good 8x12" print.
A Class 4 or 5 sensor makes a good 12x18" print (no common film equivalent - some odd 35mm semi-panoramic formats or 4x4 cm superslide, perhaps?).
645 film is somewhere around Class 6 or 7, and a Class 7 sensor makes a good 18x24" print.
6x9 cm film is somewhere around Class 9 or 10, and 24x36" prints need somewhere around Class 9-11 sensors. I make them from Class 9 Fuji files, but the difference is very obvious next to a Class 13 Z7 file.
4x5" film is somewhere around Class 13, and a Class 13 sensor makes at least a 30x45" or 32x48" print - probably more (I've made 24x46" prints from a cropped Z7 file, but larger would require a 44" printer I don't own).
Conjecturally, 8x10" film should be around Class 17 - it should just about match an absolutely state of the art digital file as of mid-2019. Print size should be 60x90" or larger
