Miles, there are a number of approaches. Some mount all photos in the same size frames, but that gets expensive. Some galleries have frames they provide. If you are a darkroom printer, i would exhibit darkroom prints. Cliveh has offered his opinion as a hybrid worker.
IMO paper edge size matters very little, since it is most common to mount the print on mat board and use a bevelled overmat.
If you don't have much experience w framing/matting prints here's some useful info. When i started matting, i used Bruce Barnbaum's suggested sizes. Here's some info i found on line....maybe it will be helpful to you.
D Karp
"1. Here is what I do: 8x10 on 14x18 matboard (I like 14x17 too, maybe better, but have not found a 14x17 presentation case), 11x14 on 16 x 20 matboard, and 16x20 on 20x24 matboard. I have done 8x10 on 11x14 matboard, but I think it looks a bit too tight. I used to use overmats, but now cut a full window mat that leaves 3/8" around the top and sides of the print, and 1/4" or 5/8" at the bottom, to allow for a signature and date.
Here is what Adams suggests: Smaller than 8x10 on 11x14 matboard, 8x10 on 14x18 matboard, 11x14 on 16x20 matboard, and 16x20 on 22x28 matboard.
Here is what Bruce Barnbaum suggests in "The Art of Photography: Smaller than 8x10 on 11x14 matboard, 8x10 on 14x17 matboard, 11x14 on 16x20 matboard (although he prefers the proportion of a 17x20 mat), and 16x20 on 22x28 board (although he prefers 24x28). He also relates that Edward Weston used to mount 8x10 contact prints on 13-1/3x16 matboard because he could cut 6 boards from on 32x4o board. This book has a good chapter on presentation, and Barnbaum's method for mounting his prints.
2. I mount my prints with a larger bottom boarder, and I just eyeball the extra size based on how the print looks on the board. Sometimes the bottom border is noticably larger. Other times it is larger, but hardly noticable.
3. I use a laborious method to locate the print on the mat for drymounting. I roughly center it from side to side, and eyeball the bottom and top margins. Then I measure the sides and bottom with a clear ruler. The print stays in place because it is weighted with a bean bag that my wife made for me out of a very soft material. Once I am satisfied with the placement, I tack the print to the mat. This method is laborious because it every movement can throw the other measurements off. Things speed up when I do more drymounting because I am in better practice. Other photographers, like John Sexton and Bruce Barnbaum cut guides from matboard to help position the print. I tried it, but I so often trim my prints from the standard sizes that I would have to have a tremendous number of guides to be able to use them to mount prints in all of the various sizes. I guess if I had to set up an assembly line, I would figure out how to make some sort of guide.
Calumet makes a device using rulers with a zero point centered on a wooden board and a "T" square to mount prints. I have thought about devising something like this, but have not had a chance to do it yet. I think that this type of device (along with the bean bag) might be the best way to quickly and accurately mount a print"