With our gelding, we had a 3.5" bit. But someone showed us to grip a pen and put it in his mouth like a bit and use that as a guide to measure is size- corner of mouth to corner of mouth. Then you measure the pen. That is how we saw that he needed a 3.75" bit instead.
I found one that was non-pinch (corners of mouth)
I'll be back and post a photo of it
I use it for regular use, we've never driven in a show yet, so I'm not sure what kind they use.
I use a myler too, I bought from Star Lake. I have found them to be the best fitting bit for a mini but if they aren't in your budget right now I would say use the pen trick to get the exact width of your horses mouth and get a no pinch bit. I really think when it comes to bits you get what you pay for so try and get the best one that you can afford, fit is of the utmost importance. Happy shopping!!
I use these different bits on different horses. Mullen mouth liverpools, mullen half cheek snaffles, regular broken mouth half cheek snaffles, and have one horse that recently has been bitted with a copper mouth french link half cheek snaffle. When ever possible, all my bits are pinchless. Most of my horses go in a 3 1/2 inch, one fits a 3 1/4, and another requires a 4 1/4 inch bit.