I ordered size 3T-4T for my son who is 21 months old and of average size, and it worked great. The "body" suit fit him great, was just a little loose but I just used velcro tabs in the back to make it a perfect fit. The robe was quite big but I just cut the length and used fabric glue for the hemline, then used more velcro to pin up the sleeves (quite a bit) to make it a perfect fit. I am very happy with! I knew it would be a little big but I did that to play it safe and I was pleased. This costume got so much attention and it was easy and comfortable for my toddler to wear.