Use a bit of baling twine. Knot it at one end and hold that end on one side of the mouth (on the outside obviously), put the baling twine through without pulling it tight but making sure it is straight. Then pinch it where it is on the other side of the mouth, take it out and measure the distance between the knot and your pinched fingers
I begged and borrowed from friends and I tried a bit of which I knew the size. I didn't ride with it, just tried it in the mouth on a bridle. It's better to start bigger and go smaller. My horse is 16h and Irish Draught, and I started at a 6 inch, which was okay-ish but perhaps a bit big. I then went down half an inch, which seemed a bit small so settled on 5 and 3/4. He's been happy in that size, the only downside is that 5 and 3/4 bits are often by order only !
Thanks guys! I used a piece of strong and then wrapped around tape so I could write the horses name down and then cut the other end when I had the correct size!
I didn't have any bits to try with my young horse so I used a piece of clean yellow hose pipe and marked it with black marker pen. She was quite happy to have it in her mouth and the measuring was spot on.