My daughter is 19months old (34.5 inches long & 26.5 lbs) and the 18months size fits her very well. I love the thickness and the feel of these.  She uses these for night time and when we are out and about.  These gave us peace of mind when taking her on outing for the day OR when we visited Disneyland. I am confident these will contain her accidents with minimal mess. We've had these for a week and she hasn't had a  mess it them yet, but I will update my review when that happens.

BTW, we potty trained in a week using the nakedtime potty training method AND then slowly introduced underwear and then finally training pants.

GOOD LUCK!