You'll probably need either a 4 1/2 or 5" bit - if you have those sizes to hand, try the bigger one first - that will give you a guide. You can get bit measuring thingys that sit in their mouth but they are fiddly.
You could try using a bit of baler twine. Gently put it in her mouth like a bit and then you can measure it afterwards. I know a few people who do it this way and it seems to work