Baby-Pants Onesie Sizing Question

I have a question regarding Baby-Pants sizing. This is my first purchase of any snap crotch onesie/bodysuit and I want to make sure I get a good one! I've read a lot of different comments and reviews saying you should go a size down than what is advertised by Baby-Pants. According to their measurements I should get a size small, but am thinking about getting size extra small based on the comments. Has anyone had any experience with them or have any advice for me?

Additionally, if anyone has suggestions of other places I should try instead, feel free to let me know.

Thanks!

Baby-Pants Onesie Sizing Question