This particular Thirsties cover is called Duo Snap and they come in two different sizes (1 and 2). Thirsties also makes sized diapers (XS, S, M, L) that are more tailored. I used an XS and really liked the fit on my 8lb baby.
Thirsties is a good diaper cover that’s well respected and lot of people like. It’s a solid, economical choice. Their covers have a single layer of PUL and double gussets, and they’re pretty roomy.
Thirsties also makes AIO diapers and fitteds (which need separate covers) that are all pretty good.
Review of Thirsties Duo Snap gives it 5 out of 5: Great Cover Comparison Thirsties