What size should I make the expander ball for a .204 bullet? Anyone know a rule of thumb or should I just make up 3 or 4 progressively getting larger?

I wouldn't go more than .002" undersize. In that small of a calibre I might even go .001" or .0015" Bullet jackets can't be that thick/tough, so case neck tension can't be much.

.2025"+or- .0004" will do ok. Are you planing to hold your neck wall thickness from .008" to .009" or something more?