I've got a 4-14x56 on my .308 PSS. it's big!

if you're going for larger glass up front, I HIGHLY suggest you get the most out of having that up there, that means a bigger tube to pass more of that light to your eye... just to pick a number, anything over 45mm OBJ, go with larger tube.

My .223 has a smaller obj and uses 1" tube.

do you really nead a 56mm OBJ on a .243?
(not that you cant have what ever you want on it)