Place a piece of white paper on a flat surface, preferably pressed against a wall. Stand on the paper with your heel touching the wall and place a mark at the tip of your longest toe. Measure the distance between the edge of the paper and the mark you placed.
Look at the columns below in the size chart and pick the size that corresponds to the length of your feet. Since the boots run small, it's not uncommon to end up at a size that is larger than your usual size. People on the lower side of the scale tend to size up two sizes. People on the middle to the upper side of the scale tend to size up one size.
If you're unsure about your size, you can always write to: email@example.com