Google horsesaddleshop and look for their templates. They are free, downloadable and printable. Their templates are the most common in use. Follow their directions for measuring and go from there. If you want to stop her from pigrooting, put a snaffle on her and tie 2, 6' lengths of cord, one to each ring then bring them together and tie a knot so it's well below her eyes. Run these two cords between her ears and back to the D rings on your saddle. Lower her head so her neck is a little below parallel to the ground and tie the cords. This allows plenty of head movement but stops the pigrooting. If she spooks, work her hard immediately doing tight circles and serpentines and make her hustle. She won't learn the first time but do it each time and she'll figure out her spooking results in hard work, usually by the 4th time at the most. When she tries to pigroot, give her lots of rein and let her punish herself.