What are you using on your dressage pony? I am currently using a 4.5" Myler Comfort Dee and while it works, it's too wide! I tried a normal eggbutt on a 4.5" and while the width fit, the mouthpiece and rings were too big. I tried a JP french link and a double joint seems to be too much movement for her. She has a tiny little Arab pony mouth and before I start ordering dozens I'd love some suggestions!
We went Stubben as they make a slightly narrower 4 or 4.5 etc than other companies & that size just "worked", nice workmanship & lighter than the alternate branded comparable bits, tack shop had versions with smaller/larger rings/D's
My pony seems happiest in a 5" when I use loose rings, but our other two ponies wear either 4.5" or 4.75" bits. My favorite is the Happy Mouth snaffle with center roller--I have it in several styles, including a loose ring, d-ring, and Pessoa two-ring (I use it with 2 reins for fox hunting). I find that if a center roller or French link is not "stable enough," pairing that mouthpiece with a d-ring or eggbutt style provides more stability.