How would I know if I should buy a small or a large?

I'm a guy with an average size face and I bought a large mask. I knew from the first time I wore the mask that it is the correct size for me. The coverage is probably better than smaller sizes, especially around the sideburn areas. If you feel the mask is too large or too loose you can adjust the little clips on each ear loop to compensate. Mine covers all vulnerable areas of my face adequately and is very comfortable. The mask need not be as tight or well sealed as all the surgical/N95 masks we have been wearing in the past two years. It's purpose is quite different. If you have an average (even to somewhat small) size face, I would go with a large.

Dear seller or user, if i wear this while swimming is it still breathable when wet?

It has a small hole for the nose. Suppose one could make a slit for the mouth. Very good quality and soft material. Worth buying.

La Necesito en color negra o gris, donde la puedo conseguir?

At this time the Aura Blue Heather is the only color available. We will be restocking additional colors over the next few months however.

Definitely washable. I've cleaned mine several times with a mild soap and let it drip dry. The material appears to be resilient and should wear very well.