Order your pads 2 inches more than the collar measures.
Example - If your horse, pony or mule's collar measures 20 inches at the front, order a 22 inch pad
Pads are available in 1 inch increments

For hames: Hames are available in increments of 2 inches. Farm style hames are adjustable at the top in 3 different positions.
Hames are sold at the largest measurement, therefore when you have a 20 inch collar measurement, you would want 22 inch hames.
In this case, the same hames would fit down to an 18 inch collar (at the smallest setting)

However, if you had a 21 inch collar, you may want a 24 inch hame. The middle adjustment would be 23 inch (2 inches larger than the collar)

The adjustment on the hames will also determine how much the "hame balls" protrude over the collar.