Here's how to tell if your boots have the proper fit:
From either a sitting or standing position, you should have to apply a firm pull to get a boot on. If the foot slides in too easily, the boot is too large.
After your foot has popped into a boot, the instep must fit just right -- not too loose and not too tight. Your salesperson can grasp the instep between his or her thumb and forefinger, then run the thumb over the leather. The thumb should slide over with only a slight ripple. If the leather bunches, the fit is too loose. If the leather doesn't move, it's too tight. Snug is the key word here.
When a Western boot is new, the heel should slip slightly on the foot. As the sole flexes with wear, the heel should have room to ride up a bit. If your heel doesn't slip, you're bound for blisters.
The ball of the foot should hit at the widest part of the boot sole.
Stand up and press your foot forward into the boot. There should be no pressure on your toes, inside or out. Make sure there's ample room for the toes to spread out.
Source: "Tips on Selling Western Boots" by Wrangler Boots.