What Size Boot Should I Buy?

Boots need to be fitted a little more thoughtfully than other footwear. Waterproof and water resistant boots are less breathable, even the ones with "breathable" membranes that are porous to air only allow a fraction of the air movement that conventional boots do. That's why waterproof boots need to be at least an inch larger than your foot to allow for more air circulation out of the top of the boot. This will help prevent sweat from staying in the boot. Another part of the moisture transport system is a thick sock. A thicker sock will allow sweat moisture to travel towards the outer side of the sock at a more controlled rate and over a larger surface area to improve evaporation. Different sock materials are better suited for different applications, but Merino wool and Under Armour socks are popular natural and synthetic options.
Boots that are not waterproof are also inherently less breathable than other types of footwear. Larger-than-usual sizing with regular boots has similar benefits. There's an odd myth that you should get boots a size smaller than usual because they might stretch. This is never a good idea. Yes, leather stretches over time, but never enough to compensate for appropriate sock thickness or the natural swelling of feet during activity. Trying to fit your feet into sneakers or boots that are too small is uncomfortable and unhealthy. Feet are miraculous pieces of engineering. The added stresses of poorly fitted footwear can throw off the balance of your foot's bones and ligaments, and throw off the balance of your entire body. Not sure why some people do it, but if you're trying to keep a proportioned look and are worried your feet look too big, you're better off opting for a less bulky design. Slightly wider pant leg ends also help to keep the size of your shoes proportional to the look of your outfit.