A longer answer…
There is a relation between needed resolution and viewing distance. For 20/20 vision, the resolution limit is around 360 PPI, at 25cm/10" viewing distance. If we assume that viewing distance is 50 cm / 20" the resolution limit would be 180 PPI. For large prints, 50 cm would be a close viewing distance.
It is often said that 180 PPI is needed for a very good print.
So, you can divide the pixel dimensions of your camera and get a maximum print size for 180 PPI.
Here are some figures:
|SLT 99||24 MP ||22"x33"|
|P45+ (older MFD)||39MP||30"x40"|
|IQ350 (50 MP MFD)||50 MP||34"x46"|
|IQ3100MP (highest resolution MFD)||100MP||48"x64"|
The table above is based on 50 cm viewing distance and 20/20 vision. I practice you can print quite a bit larger, if proper sharpening is applied. The reason is that it is more medium and low frequency detail that dominates visual acuity.
I made an interesting test a few months ago. My Distagon 40/4 CF on the P45+ is not very sharp off axis and the Canon 16-35/4L zoom I use with the A7rII that I have is significantly sharper.
I made two A2 size prints from 50% crops, that is something 33" x 47". There was no discussion that the Canon/A7rII was much sharper in that region the Distagon/P45+ combo at 50 cm viewing distance. The Distagon/P45+ was actually below what I would call acceptable at 50 cm.
But, moving back to 80cm - 1m the difference disappeared fully and the Distagon/P45+ image actually looked a tiny bit sharper.
How come? The central part of the image was actually quite sharp with the Distagon and it may be that it had higher contrast for medium detail.
Here is an off axis crop from the Canon 16-35/4L at actual pixels:
And here is a similar crop from the Distagon 40/4:
The weakness of the Distagon can be explained with the MTF data from Zeiss attached. I have seen a similar, but less pronounced loss of sharpness on all Hasselblad Distagons (40/4CF, 50/4CF and 60/3.5 CF).