The vest is cotton, so it breathes but I've never actually worn one out in the heat. The measurements are:

Small - Chest size 36-38
Medium - Chest size 40-44
Large - Chest size 44-48
X-Large - Chest size 50-52
XX-Large - Chest size 54-56

We recommend a size larger then you would wear to give the pockets some expanding room.

Andrea