T-Shirt sizing questions
 

So, I really want the A-Nation for Life Team Message shirt.

http://www.yesasia.com/us/ayumi-hama...0-en/info.html

This is an XL, but I don't really know what that means. How is Japan's shirt sizing? Is this particular shirt equiv to a women's xl, a men's xl, or a junior's xl in US sizing?

Metric measurements can be seen on the page (you can use an online converter if you're more familiar with inches/feet):
【サイズ】　XLサイズ：身幅約530mm×身丈約730mm×袖丈約200mm
Asians tend to be smaller than Europeans/Americans, so the sizes there are also smaller.
A shirt that measures 730mm anywhere is big enough to be a nightgown for me, but I'm around 5'4 and 110 pounds. I don't know your measurements, but take measurements of the shirt you're wearing and compare.

I tend to take it one size down for US sizing... that XL there is pretty close to a Men's L here in the US.

Yup XL is like Giordano size L in Singapore