37' total, with 10' for the short side. But you'd be better off with a two piece, or maybe an ATW pattern. If you do a 1-piece on the 300, you'll wind up doing the crosses bottom-to-top, and that's not usually a good idea.
I string my M-Fil 300s with 33' of string. Note that I'm using a linear gripper. With a rotational gripper, you might need about 35'.
I've never really measured out the short side as I always pre-string the mains before tensioning so that I don't waste any string. I will then clamp the long side centre main before undoing 3 mains on the short side, then start with the stringing and tensioning.