I don't know where some of you guys get the idea that there are #10 and #11 nipples. There are various makers of after market nipples and then there are the nipples installed by the manufacturers who produce the replica firearms of the 1800's. The ROA is in a class by itself. What there IS are various dimensions of percussion caps produced by three major manufacturers of percussion caps and I believe there is an Italian producer also, don't hear much about them anymore. The caps normally used here in the US are Remington's, CCI, and German RWS 1075's. All three of those outfits produce caps that differ internally and externally.
Remington #10's differ than CCI #10's. Remington #11's differ than CCI #11's........... RWS 1075's are similar to CCI #11's............. Rem 10's, CCI 11's, and RWS1075's are the best for a high 90 some percent of what after market and original nipples found on replica revolvers with most of the opinion (and mine) that Remington #10's are best overall. A few small caliber firearms will take CCI #10's which are the smallest initially available cap, they don't fit well on most larger revolvers and nipples. If readers of this forum who are new to the game would do a search, there are threads and threads speaking of this.
I'd suggest you go to the 'Shooting Accessories' section of this forum and check out a thread titled 'CCI Caps', read esp the #2 post. It deals with what you are asking and read a few other ones, there is a informative chart giving the internal and external info on caps available. One of the rules of percussion shooting......... 'Match your caps to the nipples you have, don't try and match nipples to caps. In case OP you are talking about caps and nipples for a percussion shotgun or rifle, the same idea and rules apply. CC