I use 5/16". It seemed awfully small at first, but it runs through the blocks like butter and isn't that bad on the hands. Depends on the type line you get.

7/16" and 1/2" are too big.

3/8" works OK, but it depends on the type of line you use.

What's your budget? Sampson Trohpy braid is a good all-around line that's easy on the hands. About $1.13/ft for 3/8".

Or, you could go to the Cadillac of lines - Maffioli Swiftcord at $2.87/ft for 3/8"

Lots of stuff in between.