The following are printable sizing charts for boots. Its best to measure your foot at the end of the day (most swollen). If you are between two close sizes like a 8.5 and a 9- go with an 8.5 as the boots need to be snug as possible. Leather will stretch over time.
If you decide to go the old-fashioned route and measure your foot- make sure its in the evening and that you are standing on the ruler to get the most accurate measurement. To measure the width, use a tape measure and tape all the way areound the the widest part of you foot (just below your toes).