I am new to the business side of photography and have decided to give clients the option to buy digital files with printing rights. I obviously don't know what sizes the client is going to print these in so how do I size the files to give to clients? I was sizing them as 4x6's as that would allow for cropping if they wanted a 5x7 or 8x10. However, a client then had an issue with resolution when she tried to make this digital image that was cropped at 4x6 into a 11x14. In the past I have sized them 8x10 but then when clients went to print smaller sizes the cropping wasn't good. So I'm stumped! Should I give two copies of each digital image? One for smaller prints and one for larger prints? Also, if I set the resolution to 300 dpi for all the pictures will that give a nice clear print for any size?
Why don't you ask your client, what kind of size she/he wants ? It'll her/his choice and responsibility for future prints.
If the images are of your clients or commissioned by them, and you are charging them for digital files, just give them files at the original resolution. There is really no such thing as sending too many pixels to the printer regardless of final print size.
The whole thing about DPI when choosing the pixel size of an image is misleading, but if you want to guess what print sizes are possible from a pixel dimension, try:
pixel dimmension / 160 = inches of OK printing
Size in inches and dpi are meaningless metrics for digital files, only the pixel dimensions are relevant. If you do not want to simply sell the images at their native resolution, you could provide several options, say 2, 6, 8 or 20 megapixels, for increasing prices, of course.
That way, it becomes the customers decision how large they need their images to be.