Sit in a straight chair with your knees bent at 90 degrees to the floor.
Wear the breeches and socks that you would most likely wear with your half chaps.
Have someone else measure your calves, if possible:
A. To determine calf width: Measure the calf width at the widest part of the calf. B. To determine calf height: Measure leg height from the floor to the bend behind the knee.
On initial fitting, chaps should fit snug and will give over time. Depending on materials and construction, fit may vary slightly across styles.
I have wide feet and this were a little snug the first few days of wearing but not too tight. Now after a bit of wear, they are exactly what I was hoping for. Perfect fit and so comfortable, no blisters and great durable leather