The rings of the martingale should either reach the gullet of the horse or, on one side, up to the withers. You should be able to slip and hand between the body and the neckstrap.
The part that goes around the neck, you should be able to four fingers in between. The rings that the reins go through so reach the bend of the neck by the cheek bone and the part that the girth goes through, again, I believe you should be able to fit 4 fingers in between.
I may be wrong though!! But this is how I would fit it!