If I remember correctly, a small shank nut is 7/8" long and a large shank nut is 5/8" long.
Large shanks are most commonly found on the WSM's, and on Precision Target Actions. The only long actions were the short run of RUM's that haven't been made in several years.
I'll also add my own opinion. You don't need a large shank for anything RUM sized and smaller. If you want to go larger sized, you should have the barrel fitted with a shoulder.
I've built a ton of WSM's and RUM's on small shank actions with no ill effects.