Now I am not in a position to personally wear a condom (male condom, as intended) but since I have witnesses a teenager pulling one completely over his head I feel that they contain enough give to really make this sizing thing unnecessary. Of course, people put pantyhose over their heads, and I always have to go up a size from what the chart on the back recommends so perhaps I simply don't understand. But apparently, if bored enough, one could wear over 600 condoms (as intended - NSFW*). However, if sizing makes you or your partner happier (pun not intended), then go for it.
*I haven't checked the link that this link leads you to, since it is NSFW and I haven't logged in a home to peruse it .