I used to think that "bigger is better," until the first time that I saw Bill Schwab's wonderful little 7x7" prints. I then began to appreciate the value of small prints that invite the viewer to stand close and study them in detail.
The largest print size that satisfies my eye is 11x14 from 4x5 negatives. I do everything in my power to produce negatives so sharp that they cut your eye when you look at them through a loupe. I also like to look at tack sharp prints, which I can reliably make at 11x14. I recently made some 16x20 prints on commission. They were good, but to me, not as satisfying to look at as 11x14 prints. I also noticed that the degree of difficulty was subject to the inverse square law as the size increased.
11x14 prints matted out to 17x21 are large enough to command sufficient attention with the paintings in the gallery to which I belong, yet are small enough to encourage the viewer to get close enough to contemplate them.