How to Size Long Underwear Tops?

[Irulan]

Then why do none of the size charts ever indicate this. I knew I was supposed to take a size down, but they should put something like "if you typcially take a large, size down to a medium."

I personally have never heard "size down". Baselayers are designed to be worn next to the body.( And I have worked in the technical clothing/outdoors wear industry for 15+ years) It's already been said that sizing will vary from manufacturer... but if someone is suggesting to size down perhaps they are looking at using "non-baselayer" items as base layers. There's nothing wrong with that of course.