INITIAL FEEL
When pulling on a western boot, as your foot makes the turn into the foot portion of the boot, you should have a little resistance and need to pull slightly. You should then feel a slight pop as your heel pushes down past the heel counter into place.
When trying on a pair, make sure you are wearing socks that you would normally wear to account for fit. Keep in mind that feet tend to swell during the day, so it's advisable to try on boots later in the day to find the most comfortabe fit.