Copy and Pasted from my hosting site. Hopefully this will give you a little in-site.
Suppose your photo is 2000x3000 pixels and you expect your admirers to order anywhere from 6" prints to 40" prints. What should you do?
Our recommendation is to leave it alone. EZ Prints will upsample/downsample as needed and they can do it better than all but the most serious experts.
We can feel your worry and doubt: "But...a 30-inch print is only 100 dots per inch..." We routinely show the following print at 80 dpi, 24x30 inches to passionate and fussy photographers. Their only comment about quality is "stunning":
http://cdn.smugmug.com/img/help/high-resolution-photo-storage.jpg
Photoshop books and forum posts tend to focus on resolution, compression and using color spaces which capture the broadest range of colors.
But buyers of prints give dramatically different reasons for returning them.
The lesson of millions and millions of prints:
The overwhelming reasons prints come back are skin tone, under exposure, and contrast. Choosing a broad color space hurt—not helped—the return rate and was responsible for 5% of returned prints.
The big surprise is not even large prints bound for museums or photography exhibits had issues with resolution or compression.
http://help.smugmug.com/customer/portal/attachments/6304
After reading all that, get ahold of the original file, and see where you measure up with this chart. These are minimums.
(If you can't get a hold of the original I would find something else to do for him)
http://freezeframephto.smugmug.com/photos/i-rF38jPb/0/XL/i-rF38jPb-XL.jpg