These are unisex, so all genders can use their general size. These sizes are for a fitted look- but not skin tight- that's why a 36inch t-shirt is recommended for size a size 8 whose actual bust measurement would be about 32inches.
Often, customers want a loose fit- most of the models are photographed in the next size up, so show a looser fit- so if you are a fan of the baggy comfy vibe, why not go up a size.
Feel free to just purchase and write a bust/chest measurement in notes with a short description of how you'd like it to fit, and I'll message you if I think you'd prefer a different size.
Most designs can be requested up to 5XL if you need, it's best to send measurements and fit preferences for larger sizes, as body shapes can create different fits.