Since most of my snapshots are from 35mm, my albums are all for 4x6 prints, so I have everything printed to that size. The places out here basically just crop the photo to cover 4x6, or fit it the frame on the paper (leaving a couple bands of space).
When I start printing my own, I will still do 4x6, but may trim the paper square, and just put it in the 4x6 sleeve of my albums. I basically print (have printed) every frame with an identifiable image, even if it's blurred, has horrible exposure errors, or is otherwise messed up. Everything goes in.
This is only for the albums, though. A nice photo that I wanted for presentation would be enlarged to 5x7 or greater, and perhaps cropped for composition.