Trace an outline of your foot (remember to hold a pen at the right angle to the floor). Measure the longest distance (between the centre of the heel and the tip of the big toe) and compare the result with the size chart. Do it for both feet - they may vary a little. Remember that insole cannot be shorter than your foot.
All measurements in the size charts are examples and the actual measurements may be slightly different depending on manufacturer, style or purpose of use.