I have many 16x20 prints on 20x24. Sadly. Way too small for my taste, but at the time I could not afford bigger as I could get two 20x24's out of a full sheet (32x40). Image size is around 15x19, with a window cut about 16x20. I was printing full-frame 4x5 negs.

I found that 22x28 is great for vertical 16x20 prints, but lousy (IMO only of course) for horizontals. Way too much space on the sides compared to to top and bottom. Might work fine for full-frame proportions of 35mm negs.

I find that 24x28 looks great for both vertical and horizontal 16x20's. Again, image size is actually around 15x19, with a window cut just a tad bigger than 16x20. Love this as I get a usable 16x20 piece for 8x10 or 11x14 images!

The 24x28 requires more wall space than 20x24, of course, but I like the amount of white (~4") around the prints.