Large dunk n fluffs are a snug fit on my little one but we have an XL which fits well. We didn't get on with size 2 lls so not much point in trying size 3 for us!
I was just thinking my DS is obviously very skinny. Have you tried Diddy Dippers not sure how the rise would be but because the "arms" are nice and long they would nippa easily. I have one you could borrow to try if you wanted?
Personally I'd re-visit the potty training, but try toilet training instead - scrap the potty and see if she will copy you or copy an older child and use the toilet.