i bought the arm coolers, they told me i needed a small, so i trusted their judgement and they fit fine. Honestly, it hasn't been really hot since i got them, but i did wear them in the sunshine two rides worth and i didn't use any sunscreen nor did i get any itch or burn. so they seemed nice and cool and they fit fine.

I'm sure there will be at least 1 hot day in the next few months.