IT all depends on where you like to have the placement of your string at full draw for me i like for the loop to be rite at the corner of my mouth at full draw and my arms are not fully exstended so the chance of my string hitting my forearm are very slim i dont like to have to bend my arm to keep this from happening if you have to do this it means you are not actually getting the full draw out of your bow because
you are letting the string go forward so you can get
that bend in your arm but thats how i judge mine i want the loop rite at the corner of my lip so the loop size will vary deppending on how and where you like your string at full draw.good luck and happy hunting.