Yeah, I'd say they run a bit bigger than, say, Abena X-Plus or Bambino. I wear a medium in both of those, and the medium 24/7's give me plenty of room, but without feeling like it's too big.
In terms of being discreet, I would definitely recommend xp medical. It will only have your name, and for the return address it just says "Gary Evans XP". XP Medical has great customer service as well. I received one sample pack of the 24/7s that had a tear down the back of it, I reported it and they replaced not just one brief, but the entire 3-pack sample (great especially in comparison to "the #1 ABDL diaper" company's customer service, who when I reported a major issue with the tapes on the Teddys only offered to replace the sample pack if I purchased a case - really?? Two of your diapers had horrible tapes, and I'm going to want a whole case? I don't think so.)