Really depends on the watch (dress vs. sport), but between 38 and 42 is ideal, with 39/40mm being the best.
I have a 7 -7.25" wrist
Thickness is very important in wearing comfort.
For instance, I could pull off the 45mm Blancpain Fifty Fathoms, but the watch is ridiculously thick IMO. On my wrist is just doesn't work. Watch companies should focus more on reasonable thickness.
Even the JLC deep sea chronographs, as another example, seem thick to me...nearly 14mm for a 40.5 - 42 mm watch is getting to be too much, especially for the former as it begins to look and feel unbalanced.
Cheers,
John