I have a very small 2yr old (he was a premmie) and bought the 2T-4T size. It is WAY too small for him! There was no warning to buy a larger size and I did check the size chart, but he is still in 24mth clothes so I thought this would be too big if anything. The costume did arrive promptly and is very cute.
I called the company for an exchange. I have to rebuy the costume, including paying the shipping again, then also pay to return the costume. In all it would cost at least an extra $10-$14 just in shipping costs when it was not my fault to start with, plus I have to pay again now and hopefully get the basic costume price refunded later.
There is no parent who would think of ordering a size 4T-6T for their tiny 2yr old!
So my recommendation is to either buy an extra-large size first or buy from a different store with better options for exchanges.