The common wisdom is to hold your shooting arm out to the side level with what you're standing on. Then bring your forearm up at a 90% angle toward the sky. Measure the distance from the inside of your elbow to the second knuckle on your trigger finger. That should be your ideal length of pull. Well, maybe. Many of us like it shorter or longer due to our personal preferences. The above will give you a place to start.

Go tell the Spartans,Travelers passing by,That here,Obedient to their laws we lie.

I'm older now but I'm still runnin' against the wind