I'm a huge fan..
Yes, size up. I use mine in treestand hunting probably 15-20 times a year. For that application I've found the very best recipe is to walk in with thin merino socks, uninsulated boots. Then once you're set up at the tree, change into thick DRY wool socks, your uninsulated boots, drop a handwarmer in the blanket and put them on. I can sit 13 hours straight, in sub freezing temps, with my feet on a metal platform NO PROBLEM doing this. I used to suffer from cold feet using insulated boots,, but no more. This is the key to warm feet all day long.