The largest I've bi-cubic, smoother up-rezed in Photoshop CS3 and newer and it looks great at even 18 inches is a 10 MB capture with a sharp lens to a DX sensor.
The print hanging above my desk is 30x45 inches and is just fine. It's printed on an Epson 9800 at 180 dpi on Luster paper. I simply told Image->Size that I wanted the file to be 5400 pixels on the short edge and about 8100 pixels on the long side from the 2600x3900+/- original capture. More than likely, I did the first up-rezing in ACR to 4100x6100 then the second up-rezing using Bicubic, smoother in PS. I finished the image by running a sharpening action created by Lewis Kemper years ago to finish the processing.
Doing the math, the image should only print to maybe 9x13 inches, but I wasn't all that good in math at one time;-)
If the image is exceptional, shot with an optimized system with the proper settings with a sharp lens and your technique is great, you can really push the limits of a digital file. Some have pretty good luck with Genuine Fractals, but I haven't found I needed to use this program in several years and most people that use it, say it's very close to what you get with a simple up-rez in PS.
If the image won't be looked at by the anal-retentive at 18 inches (like me) and is printed with a good printer on, say, canvas or a fine art paper, you can even get larger prints than this.
If you are concerned about the image quality, simply up-rez the image and print a section on letter-size and see what you think. If the section looks good close-up, then the image should work large. If it's "iffy", try putting the image on the wall and step back. It will probably be OK.
On a job I'm pitching to a local client, I've got several 20x30 prints and a couple of 28x42 prints that will be printed on canvas. The sample prints up close are so-so (my 18 inch test). At three or more feet away, still too close for proper viewing distance, they are just fine.
Of course, your results will probably vary. Try printing a small section and see what you think.
