I don’t know if this will help but it worked for me. I had an FEI dressage horse that was an OTTB that had the funniest head. He was an 4.75 bit, almost oversize length of his head and a cob width. Since he was really good at what he did, I bought him everything he needed. This same horse also evented though Prelim and went in 10 million bits and could and would wear bits that were too big for him (He could be a handful XC). For dressage, I had a double bridle with bits that were too big because at FEI they have to wear a double, but I mostly rode in my snaffle schooling. I FINALLY found a bit that fit him… a 4.75 bradoon (the snaffle part of the double bridle) since he loved his plain egg butt snaffles. I would just use this on my plain snaffle bridle with no other bit. These bits are made to be smaller as to also fit the curb in their mouths. You can just find them on ebay, not easy to find but doable. Probably will be under pony double bridle wear.
I swear he went better in that than the double or anything else! As long as he was in the arena