Put a saddle on her and using a piece of bailing twine put one end on the 2nd hole of the girth strap and do it up like you would a girth on the otherside. Mark with a pen or keep hold of it. Take it off the saddle and measure.
Or same thing but with a weigh tape, whatever you have to hand.
Otherwise, ask to borrow someone else's on your yard just to get a size idea.
It's hard to just say without seeing the pony or even which type of girth you want.
haha, this has made me smile..when I tried to measure the boy for a girth, I came up with seven different measurements - hubs was NOT amused. So I borrowed three sizes, and then decided from that. And I now use the girth as a weigh tape, depending on which hole he has to have