I was a little worried about what size to get (my usual size S, or to size up to a M). I ended up getting the M because I have a larger bust and big thighs and with some fabric, if it doesn't have enough stretch, it digs into my thighs. But really there was no need for me to worry at all. This onesie is so stretchy and the fabric is light weight so you can wear it as is in the hot summer sun or pair with a jacket if it is chilly. So even though I got a M and it fit amazingly, I probably could of gotten a S as well and either could of worked! Another plus is that the bust is lined so you don't have to wear a bra if you don't want to (if you are like me and never wear a bra), and also the straps are adjustable to make the onesie fit perfectly to you body. Seriously buy this if you have not yet! I am planning on ordering the leopard soon and am praying HAH comes out with more colors so I can add those to my closet as well!