It's hard to find a true size for your needs. I for one wear them as underwear for the pure purpose of the feeling overall the desire to be in them. I tend to wear a large in fetwear which gives enough room to play in after a long hard day. I've noticed from many brands the leg holes being cut too wide or too narrow and that seems to be the issue I'm faced with a lot.
fetwear has some nice trimfit bikini styles for those who just want to wear them as underwear or a smaller padding underneath. their material is also very soft and smooth inside and out and isn't rough like those cheap pants you see all over the internet.
pvculike makes some fantastic panties but they are a bit more expensive. What I really enjoy about their pants is the elastic is encased in the plastic and they give supple stretch and leg room. you can order them form makeitkinky who is in the USA with free shipping.
if anyone has questions please let me know as I own well over 50 pairs, different brands, have tons of items to share (bedding, pjs, sauna suits, skirts, wife has babydolls, lingerie etc all plastic from various makers)