Check out this awesome cropping guide. It will definitely help give you an idea of what your picture will look like depending on the size ordered. Of course my recommendation is to always go BIG if it's for wall display. The idea is to see your print not the wall!
Many of the older frame sizes use cropped images. Cropping is basically removing part of an image to fit a size. If someone is interested in using frame sizes such as 8x10, 16 x20, 24x30, they would have to crop the image to fit. When I shoot, I don't leave much room for cropping in my photo's. Cropping could negatively effect an images final look. I always suggest to my clients to use the sizes that are truer to my camera's final digital sizes. I suggest that clients choose: 4x6, 5x7 (slight crop), 8x12, 12 x 18, 16 x 24, 20 x30 and on.
Clients can get frames when they purchase products thru me. Or they can always get them online, frame shops, or even craft stores. They may not be the most popular sizes, but they are becoming much more popular today.
Here is an example of how cropping can change an image (below). Again, I suggest, not to crop an image if you don't have to.
Please don't hesitate to ask any questions when ordering or regarding crop sizes. Any orders going thru my website will be checked before they go to the printers for proper cropping.
I hope the handy guide is a great help!