Please refer to these charts before ordering if you are not familiar with the brand of shirt it is printed on! The style of shirt should be identified in the description. Ladies shirt sizes can be especially zany, as many ladies have no doubt noticed in stores. The Human Bird and Paperklip even made an incredibly charming video about those.
One thing to keep in mind American Apparel, Royal Apparel, and Alternative Apparel all run about a size smaller than their Gildan/Hanes counterparts. Ladies 2XLs are pretty small, and if you were to be in the market for a Ladies 3XL, the size basically picks back up at Unisex Large. The cut is slightly different but the message is the same: you are cooler than everyone else.