The sizing is fairly adjustable - so P/C/H sizes - which ever is appropriate should be fine.
If you are meaning the length at which to set the martingale, the rule of thumb is that:
For the running martingale - when the girth part is attached to the girth, the rings should fit just up into the horses throat - where the jaw meets the neck. Ideally, when you have a rider sat on the horse, with hands in the normal position, the martingale should not interfere with the reins (ie pull down).
For the standing martingale - again, with girth loop on the girth, pull the strap straight and it should fit up into the horses throat and then follow the jaw line to end where the noseband should go.
Thanks for replies, I'm needed a measurement of the strap from head to girth. The one that was posted to me was meant to be full but on its longest buckle hole it only just meets he head and is far too short to ride. The person who sent it swears blind it's full so I need a measurement of length to prove I've been sent a pony sized.
Running Martingale - fit lower strap to girth then bring rings up the side of the shoulder to the wither, they should be a hand span below the wither.
Standing Martingale - fit lower strap to girth and top loop to a Cavesson Noseband, when fitted correctly the loop should reach into the throat of the horse.
Both Martingales should come into action when the horses nose is the same height as the wither.
Measuring to buy
Neck strap - measure around the shoulder starting where the neck enters the shoulder.
Main strap on both - measure this from girth to handspan below wither for running and from girth, up to where head and neck join down to noseband for a standing.
Remember that both are NOT for holding the head down but to prevent it from coming up too high.