When you shop for Western boots, Justin Boot Company Inc., bootmakers since 1879, offers these suggestions for a good fit:
HEEL - The foot should slip a bit in the heel. Because the boot is new and the sole is stiff, slippage is normal. After the boot is worn, the sole flexes in the ball and most of the slippage disappears. A boot that fits right should be a little hard to pull on the first time. If there is no initial heel slippage, the boot may be too tight and could cause blisters.INSTEP - The instep should be snug but not tight. If the boot slips on too easily, the instep is probably too loose and a narrower size is required. Check the fit by running your thumb and forefinger down the front of the instep. If you can pinch up leather at the highest point of the instep, the boot has not been fitted correctly.
BALL & TOE - The area of the foot that bends at its widest part is called the ball. It is important that the ball aligns with the widest part of the boot for a comfortable fit. If the boot is too short, the ball of the foot sits too far forward and crams the toes into the toe box. Ample toe space is necessary because toes tend to settle forward after the boot is worn for a time. If the boot fits right, the toes should touch - but not push against - the inside of the boot when standing.
ARCH - Sturdy arch support, accomplished with a steel shank in quality boots, is among the reasons boots are generally more comfortable than shoes. Remember that boots have no laces to hold them securely to the foot, so width is extremely important in proper fitting. The arch is a vital area in determining a correct fit. If the arch of the boot is shorter than the the arch of the foot, the area just behind the ball of the foot won't touch the sole.
In general, if a boot doesn't feel right the first time, try another size. Beauty and comfort are partners at Justin. You should never sacrifice one for the other.