We recommend sizing off of the Brannock device (any shoe store will have one). If you want a snug fit similar to a dress shoe, go with your Brannock size. If you prefer a bit more roomy and wearing it with boot socks, size up by half a size. For example, Mikhail is an 11D on the Brannock and wears a size 11.5D Bliskavka boot.
For wider fittings, go up a half size. I.e. a 10E on the Brannock would be either a 10.5D for a snug fit with dress socks, or a 11D for a looser fit with boot socks.
Note - The leg opening of the boot is designed to intentionally be snug. As long as you can close the zipper you’ll be good. The leather will stretch and loosen up around the top with wear. You may need to use a little muscle when trying on your shoes for the first time. A shoe horn can come in very handy and is also a great tool to use to ensure the longevity of all of your shoes as it prevents the crushing of the heel.