You can order any size you want from JT ball molds. They were pre-2020 about $42 shipped to my door in about 3 weeks. I don't know if it is still true.
https://www.ballmoulds.com/
He cuts the molds to the CAVITY size. I find on average, they drop balls about .003" smaller than the cavity size. I've got 5 or 6 of these molds, and they are usually pretty close to that. For a .630" ball, I'd recommend a .633" mold. For a .625" ball, I'd recommend a .628" mold. It's not rocket science with these, which is what makes them so great. As long as you are somewhere in that .002" to .010" over groove diameter, you should be good. Slugging your bore is best. If you measure with a calipers, the reading always seems to be smaller by a couple thousandths. If you measure with a calipers and get somewhere in the .614" to .618" range, I'd get a .628" mold. If you are somewhere in the .618" to .625" range, I'd get the .633". One other note, I'm giving these one or two coats in ALOX tumble lube for good measure. I get zero leading.
Bloodtrail won't lead you wrong either. He gets some real impressive accuracy with those Russian slug molds.