I bought the XS for my 4 year old son, he needed a sweater for cool days in Houston, it fit him perfectly and it's a medium size for a 4 year old. I went ahead and bought the small which is a little longer and better for the fountain pen and the XS fit my 2 year old and surprisingly it fits perfectly. If you want a true to size fit then generally stay the size you wear but if it is for a growing child I would go up a size. Also gave four stars because of the plastic zipper, apart from the fact that the sweater held up well after the first wash but I didn't dry it.
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