I use 24x28 (actually 600x700mm) for my 16x20 prints. Window size is about 15x19.

You do end up blowing most of a 32x40 mat on each frame with some left over for framing 8x10s, but that's still way cheaper than the museum glass etc. Assuming you're not randomly changing the aspect ratios and can live with a standard 15x19 window, you can of course re-use the mat and change the print over occasionally.