Optimum diameter is always a close slip fit to the cylinder throats, which means a sized diameter of .4525 for your gun. Order exactly that size and I guarantee at least that diameter, and not more than .0015 oversize. Sized diameter will depend on bullet hardness while sizing and actual sizer diameter, as sizers aren't held to the closest tolerances. A 452 sizer, if actually that size will produce your necessary diameter if bullet hardness is about 16 bhn. At 20 bhn you may have trouble chambering, because sizer die spring will produce diameters a bit larger. At 12 bhn, bullets will come out of the sizer at exact sizer diameter, at room temperature, slightly smaller if using LBT lube in temps close to freezing, because the lube stays between bullet and die, causing it to size a little undersize, but not if hardness is at least 20 bhn.
Did I throw you enough variables? - To be sure you get a sizer that works, order both 451 and 452, if for Lyman or RCBS, or get a 452 if using a Star, as Star tolerances are held tight. If necessary, the 451 die can be polished out to produce exactly the diameter your gun needs.
