I am 5'5" and 125 pounds.  I purchased a Large. Fit perfectly.  Very good quality. Nice thick stirrups on the legs and a thumbhole on each sleeve.
Only reason I am exchanging it is that the ORANGE in the picture online is PEACH.  But in reality, it is a bright neon orange......so I'm going to exchange it.  Too bad because I would have LOVED it in peach, and I purchased swim shoes to match the peach.