You will find a useful bit measure you can cut out and create. If you print it out on paper, you'll need to stick it to some thick card otherwise its too flimsy.
There are some really good tips on the site too about assessing the shape of your horses mouth and deciding from there which sort of mouth piece is going to suit him best.
An easy way is to get a wooden spoon and two rubber rein stops (the things you put on your reins to stop a running martingale getting tangled in the buckles) You can use rubber bands as well but I find the rein stops move more easily. Put the rein stops on the handle of the spoon fairly wide apart. Put the spoon handle in the horse's mouth so that one rein stop is pressed up against the outside of one side of his mouth (with the other one on the other side of his mouth) and then slide the other rein stop along the spoon handle up to the outside of the other side of his mouth. Remove the spoon and measure the distance from the OUTSIDE of the rein stops. Get the closest size to this measurement - if it's between measurements, get the next size up.
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New horse is coming soon and I need to get tack sorted is the best way to measure a bit with twine?
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Why not just ask the Vendor if you can buy the bit which your new horse is currently wearing?
If they won't sell, ask the Vendor if they could possibly lend you the bit for a couple of days - then take it to your local saddler and buy accordingly. I am sure they would be more than happy to know that you are concerned about buying the correct size.