Hummmm, I dunno...
Tell ya what, measure the outside diameter of sized brass neck, then the neck thickness of the same piece of brass, and the diameter of the (I'm assuming) cast bullet, and maybe we can figure it out...
Or, if you trust yourself in measuring inside diameters, measure the inside diameter of the neck of a piece of sized brass...
Or measure a piece of peeling with a micrometer...
Or bell the mouth of the brass and don't worry about it!
If you're talking about jacketed bullets, that's another story...