This is a good reason for not giving customers images on a disc. If they are not happy with the print quality, or try to print a different aspect ratio, they think the photographer screwed up.
A good 8x10 can be printed at 100 ppi, if the print is made from a high quality photo.
The 300 ppi rule of thumb is outdated and remains from the early days of digital imaging when top grade pro cameras had 2 - 5 MP.
The math equations:
Pixels / inches = PPI
PPI x Inches = Pixels
Pixels / PPI = Inches
Another thing you can do is limit the file size by lowering the Quality setting when you save the image file.
Print size limits can be included in the use licensing (print release) documentation for the image.