5x7.

4x5 just doesn't seem that large to me anymore, and 8x10 is a bit much to carry around. 5x7 or 4x5, a few holders and an extra lens or two, both fit in a messenger bag. Also, 5x7 is like looking at a real picture on the ground glass in that the viewing distance under the focus cloth is just right for 5x7. I can't get far enough from 8x10 outside to feel like I'm composing the picture as a whole unit, but it's OK in a dark studio. 4x5, too small again. Also 5x7 contacts are sweet--the size I always printed 35 mm anyway. Again 4x5, not so much--too much like prints from the drugstore.

Thus, 4x5 or 5x7 on the run, 5x7 or 8x10 in the studio; 5x7 either: perfect.