Sheave size is mostly about wear to the cordage. Groove shape also. My Original Kenyon sheaves are "V" shaped, probably able to accept a wider range of diameter size. For the forces involved a very good 1/4" would likely handle the strain, but your fingers would be bloody stumps. 3/8" slightly better. With 7/16" I get enough hand grip so I can haul the main up as high and as quickly as my back will get it. The small remaining haul is by winch. Quicker = less flogging. I had a Sta-set X halyard and did not like it. Very stiff and hard to handle. The majority of us will do just fine with regular Sta-set and save a few dollars.