The selecting of a Bosal is a personal choice and varies depending on your horse-man-ship, views, goals and experience.
Here are just a few simple guidelines that might help you with your decision.
If your starting your horse or he has been ridden in the snaffle and you're looking to stay out of his mouth, I recommend a Bosal size 5/8" to 1" in diameter.
This gives the horse a clear feel and a quick release.
If you have been riding him in a hackamore, he is going nicely and you're looking to advance him further before progressing to the 'Two Rein Bosal', I make Bosals from 7/16" - 1/2" in diameter.
These pieces are more refined and give the horse a smoother transition to the 'Two Rein Bosal'.
The 'Two Rein Bosal' is the final step before the horse goes straight up in the bridle. When used with a small Mecate and in conjunction with the bridle reins, this traditional piece allows you to preserve the horse's mouth while developing him into a finished bridle horse.
The 'Two Rein Bosal' can vary in size from 5/16" - 7/16". This size Bosal fits nicely in between the bridle rings and the horse's nose, but still offers enough feel for it to be effective when needed.
The Bosilito is for a finished bridle horse and is to honor the horse for his level of excellence. It is a decorative piece that can be tied from the foretop or used with a hanger.
These guidelines are not a hard and fast rule of what size you need, just some ideas that might help in your selection of a Bosal.
I am happy to braid whatever you would like and look forward to talking to you personally.