For the saddle, go to horsesaddleshop.com and find and download the free templates there. They represent SQHB. regular QHB, and FQHB. You download the templates, cut them out of cardboard, and use that on the horse to see what fits. If you save the piece that you cut out of the center, you can use that inside the gullet of the saddle, to test how wide the saddle is.
Bit width is measured inside the cheekpieces. It's the width of the mouthpiece. Just make sure that's wide enough to not pinch the horse, but not so wide that it slips back and forth. I never used a string. I just used bits, lol.