RevolutionRaceMen's RVRC GP Trousers, Durable Trousers for Hiking, Walking, Exploring and All Other Outdoor Activities

So i'm 5'8" and weigh 135 lbs (138 lbs at most) and have a 31 inch waist with a 32 inch length. what size is appropriate? xs?

Why there is no M size??

Are they gusseted in the crotch? Cant tell from the photos

How hot they get on high temperatures?