Short answer: the foot size of men's Wesco boots usually is true to size. For example, if you wear a men's 10D cowboy boot, then a Wesco Harness boot in 10D should fit your foot fine.
The issue of sizing has more to do with calf circumference than with foot size. Men's calf circumference (sometimes called calf width) can vary greatly, and changes with age, lifestyle, amount and type of exercise, and health.
One only need be concerned with calf width of a pull-on boot. If boots have lacing or zippers that allow for calf width adjustment, then standard widths as made by the West Coast Shoe Company (Wesco) should be fine.
Pull-on boots that are 12“ (30cm) or shorter in height usually do not have any problems with calf circumference. However, when boots are 14” or taller – especially crotch high (32“ - 34”), then calf width is a major issue. It is important to think about boots as an investment, and if you will be making an investment in a tall pair of Wesco boots, to have them made custom to fit you. That way, you will get boots that you know will fit you properly and that you will wear for many years.
As you age, you begin to lose muscle tone in the calf muscles. Calfs begin to get wider. Also, you may develop health issues such as vericose veins. Since Wesco boots will last for decades, it is important to consider specifying a larger calf circumference than you currently have. Then, as you age, you still will be able to wear the boots.
Also, if you intend to wear denim jeans or leathers inside Wesco boots, that is another reason to order custom boots. You can measure the circumference of your calf while wearing the clothing that you plan to wear with the boots on, so you know how large a circumference you will require.