Welcome @5h4993R I’m plus size, generally about 18 to 22, when I started I just got a couple, the ones slightly bigger are more comfortable but the smaller ones do give me a nice sensation. I’m not fond of crochless ones, they slide around too much. Like I said I’d just buy a couple of different ones to get the discussion going and shop around from there.
I’m not sure where you are shopping, by that I mean country but I prefer feeling rather than guessing. From there I go to Amazon and gotten some really nice ones. I like la vie en rose, surprisingly joe fresh has some really nice stuff.
I don’t wear plus size lingerie anymore but I do have a large bum and have this high waisted thong in size 16 -18:
It’s comfortable, shows off my curvy bum and covers the bits I don’t like so much! The suspender straps are removable so they can be worn as normal underwear too. Depending on your wife’s shape and style preferences, I think this is great for anyone with curves.