Sizing:
Go with the “Men’s Tshirt” option which should really say unisex bc…gender is over, right? The “Women’s Fitted Tshirt” is being discontinued but it is still showing up on the site. Ignore it.
For sizing, if you usually wear “women’s sizes”, go one size down in the “Men’s sizes”. For example, if you wear a Women’s Large/XL, get a Men’s Medium.