If the bikes is an old bike with a one-piece crank, then it may be 1/2 inch. Some pedals whether the more normal today 15 mm or 1/2" of times gone by don't allow a wrench of normal thicknesses to be used. Some pedals no longer have flats for a open end wrench and require a hex key from the back side, which screws up your idea about which way to rotate the wrench since you are now looking from the backside wanting the screw to go away from you.