To fit a boot, you want to use a foot measure. They come in Men's, Women's, and Kid's sizes. This allows you to find out what size your foot is and your foot’s width. You need to make sure that you stand on the measure with you heel all the way to the back of it because your foot will spread when there is weight on it. You can measure your left foot or your right. Men are most commonly a size 10 with a "D" width.
The foot measure can also measure the length of your arch or instep. Do note that every brand of boot will fit differently. Also everyone has one foot that is smaller than the other. Make sure that you try on both pairs of boots to get the best fit.