I have worn UGG boots for many years. For me, in the traditional UGG boot, I always go down a whole size. I do not wear socks with them and the wool lining relaxes after a few wears so there's a lot of extra room in the boot. In some of the other styles, I wear my normal size but for some reason, the original boot is cut larger. I think at one time they did suggest sizing down in that boot.
Nothing like an UGG...any others I have had fell apart. To me, they are worth it. Try and catch a sale, that's when I grab mine. Enjoy 😉