My .30-30 barrel slugs .304"/.308", so I size to .3095" (it's a .309" die, and the alloy has some spring-back).
My 7.62x54R slugs .301"/.315". Since the bullets drop at .315"-.316", I shoot them unsized.
My .380 Auto slugs .358"x.359" (don't remember what the bore was, but it doesn't matter). Right now, I have enough commercial bullets to hold me over for a while. If I were to cast for it, though, I would want a mold that dropped them at .360" or larger; and I would use a .359" sizing die. (A bullet at about .3595" is the largest I can chamber; otherwise I would go to .360".)
For my .44 Mag, I have two choices:
.429" sizer (for swaged jacketed bullets)
.430" sizer (for lead alloys)
For my .444 Marlin, I shoot certain lead bullets as-cast, and run certain swaged bullets through the .430" sizer (with spring-back, they come out at .4315"). Once my custom sizer comes in, I'll be running lead bullets through a .432" sizing die (for a .428"/.432" barrel).
Bottom line:
I try to size to .001" over bore or throat diameter. With certain alloys, I take spring-back into account (especially with larger diameter bullets, like in .44 caliber).