I've also been wondering about this. I wouldn't know which 510 connectors to purchase for my various mods.
It's a little difficult to know. There are many styles.
What I am going to do is to buy calipers and measure some of the 510s and see what they measure out at, then order the parts for those few I am unsure of. (This comment has to do with sigelei 213s (have 2) and sigelei Fuchai 213s (have 4). I have 6 total and the 510s are very small looking.
I'll probably end up with 6 to 10 - RX200S mods, and I am going to replace the 510's on them as they need it. They are faulty 510s, so I'll be buying 3 - 510s for each of those mods.