TCASuper Thermal Long Sleeve Women Running, Ski Base Layer - Compression Shirts Woman, Workout Tops for Women

I am a size 6-8 and since a lot of the reviews indicate that size guide is off i was wondering what size i should go for? size s or xs? thanks!

What is the fabric composition?

I'm a size 10 - what size would you recommend?

Is this suitable as a base layer for running in?