The easiest way to measure your horse is with a piece of string and a ruler. Make a knot near one end of the string. Lay the string over the horse with the knot at one measurement point. Find the other measurement point and hold your finger there. Keeping your finger on that point, lift the string off your horse and onto your ruler. Either standard or metric measurements are fine. After you've recorded the length, you can move on to the next measurement. That's all there is to it! Here are the measurements I like to have to ensure a perfect fit for an English bridle. Also, be sure to tell me if you are planning to use a rider doll in your set-up as it does affect the rein length.
The model featured in this entry is a customized Sandicast Quarter Horse resin named All Grown Up. She was sculpted by Sommer Prosser, customized by me, and painted by Hilary Schwafel. She's a great favorite of mine--I couldn't resist adding a full body picture of her to this post!