MY SHOE SIZE IS ?

I have recurring heel pain, primarily in right heel. I do not have heel spurs. I’m about to order arch support orthotics. I’m not a jock, so loafers are my norm. I wear size 12 Medium (I think). How do I measure each of my feet to determine exact size? Also, should I go for 3/4 length or full length insoles? I don’t appear to have metatarsial pain I do have elevated (but not high) arches. don b