I just want to mention 5x7", a brilliant compromise between the size of 8x10 and the portability of 4x5. In much of Europe the size is known as 13x18 - film and holders are almost but not quite interchangeable.

5x7" is large enough to contact print, small enough to enlarge.

Best of all, it is about the largest film it's possible to change in a changing bag, so like 4x5 you're not dependent on a mule to carry 50 extra holders - or returning to the darkroom every 6 shots or so...