My daughter is petite so I took another reviewer's advice and ordered the small even though she weighs 25 pounds. The small fit just fine. She's not crazy about wearing them, so I'm not sure how well they hold in accidents. They are fairly soft for plastic pants! I am pleased with them even though she isn't! :)

I use these covers on my 3rd child that I have had in cloth diapers. I use them with prefolds and fitted diapers. My child is 4 months old. He is at the top of the weight for the small size but as his legs are skinny I think he will be in the small size for awhile.
Even with a soaked diaper his clothes stay dry. The only leaks I have had is if I don't tuck the cloth in well. I like how soft these are compared to the vinyl type. They clean very well, I just rinse with little soap and water if soiled. If just wet I let air dry and they are ready to use for the next diaper change. I use 4 and that seem like more than enough for us.
These do not have tight elastic around the legs or waist so if you have a skinny kid make sure you order a smaller size than the weight. Just to be safe I ordered a small and a medium.
One more thought, I know how to put a cloth diaper on so I rarely have poop leaking out of the cloth. So I can't really comment on this cover's ability to hold in poop, just diaper wetness.

I was looking for an alternative waterproof cover to use on outings while my daughter is learning to use the potty so I could protect her clothes and car seat from leaks without using a cloth diaper again. These covers are affordable, soft and waterproof. They fit nicely over Gerber training pants and we have had no leaks. They are easy to wash and air dry nightly. I bought 2 packages of 2 so I would always have a spare. My daughter is petite so I chose to order the small size even though according to her weight I would have ordered medium. I am glad I chose the small for right now, they fit her just right. The Gerber waterproof covers are quite puffy, but these are not bulky under her clothes.

Comments about Waterproof Nylon Diaper Pants:

Quiet and the elastic is soft, nothing to mark skin.

My son started leaking through his diaper nightly so I needed a solution quickly, a co-worker recommended these. They work well enough but my son has been pulling the top of his diaper down while in bed contributing to the leaking so we still have accidents but at least not as many:)