I order one for a boy size 18m and another for size 2T.  They came with stickers inside the neck to show the sizes (I took them off so they didn't shed in the wash and because there are sizes sewn in on the side).  When I washed them I put the two shirts on top of each other to figure out which was the 2T and the shirts are exactly the same size (with the 2T shirt being a little small on the 2 year old).