I have a 5 1/2" eggbutt snaffle bit that I have been using for Sandie, and the other day my trainer took a close look at it and said it's too big for her (too wide). What size should I go down to -- a 5"?? Here are some photos of her that have close ups of her mouth, I know it may not help at all since it's hard enough to fit the darn things to her mouth, but just thought I'd ask, she's 14.1hh BTW...what size bit does everyone else use? Sandie face.JPG Sandie & Bailey.jpg
To find out your horse's size take a piece of dental floss (or string or whatever you have on hand) put it in your horse's mouth like the bit, and mark off from one side of his mouth to the other. Then measure that and that will be his size. If he's in between it's better to be too big then too little. The next size down from a 5.5 is a 5.25 but it depends on how big it is for his mouth. I have a QH pony that's around 14.1 that I use a 5.25 on, he has a fairly wide mouth.
A 4 1/2 is a PONY bit, a little pony bit... I would get a 4 3/4 instead. Also, when the bit is in her mouth, you should have 1/4-1/2" of bit showing on each side. It's quite common for people to have bits that are actually too small. My husban's hald arab (with a smaller looking nose/mouth) takes a 5" and the horse in my avatar (a 15.2h Appy mare who takes a cob side bridles) need a 5 1/4" or 5 1/2" bit.
You could start with bit guards and see if that makes your current bit fit ok. Much cheaper than a new bit. ;) It would be easier to tell how much too large her current bit is if you took a pic of her in it.