I'm working on a project right now and figuring out exactly what aspect ratio I want to display. I made up this cheat-sheet for myself.
Luckily I'm printing my own images at home so I can then just cut a mat to fit the size I end up with. That is, I wouldn't want to chop off critical parts of the photo (and maybe destroy the balance of it) to fit into a mat I happened to have.
What print size do you guys tend to use?
I used to do only 8x10 images (or smaller) because it's the most my printer could handle. I now have a printer that can go much larger, so I tend to prefer the larger sizes like 11x14 or 11x17 depending on the image's shape. But in general what I do is, in PhotoShop, crop the image to exactly what I want the image to show. Then I get a large frame, figure out what size leaves me about a 1.5" border on the shorter side, then cut a mat to fit. That way I'm not stuck chopping off pieces of the photo in order to fit it into a pre-existing mat. I chop the mat to fit the photo I want to show.