I like these because they are soft, they appear to run true to size. My 6 year old (who wears a 4-5T) fit into the XS until he had a growth spurt and now fits a S. He's very small for his age, but sizes seem consistent with other brands. They hold up well in the wash and don't shrink much. My only comment is that they are a bit on the thin side, the white isn't completely see through but it's not completely opaque either.
The weight and material of these shirts is wonderful. They are lightweight and do not wrinkle. I was able to put vinyl on them and it stayed nicely. Whether you want it plain or something fancy this is the shirt for you.