Most of mine are 10x15" prints (image area) framed in 16x20" frames. I also have about five frames 20x24" which I reserve for a2x18" prints.
In our dining room I have six of the 16x20's on a more-or-less permanent exhibit. Our family room has big windows along the west wall and two skylights, so there is good natural light there. In that room I have two 16x20s and one 20x24 on one end wall, and on the other end wall I have a built-in shelf about 8" deep (from Ikea) on which normally rests a single painting about 30" square. The shelf is eight feet long, so when photographers come to visit, the painting is moved and the shelf is use to display matted or unmatted prints, usually ranging from 8.5x11" to 17x22". Both of those end walls have track lights for use when there isn't enough daylight.
In my rather modest house there is no wall that would look right with anything bigger than 20x24", so I never make "big" prints (back in my darkroom days, "small" prints were 8x10" and "big" prints were 11x14").