Watching a video on how to fit a bridle to your horse, they went over proper bit fitting and that got me curious so I checked my own. I've never had issues with the bit I use but it doesn't have any extra 'space' on the outside of the mouth. The video said there should be at least a thumb length on either side of the horse's mouth.
Below is my horse and the bit she wears. As you can see it looks like a snug fit and I'm guessing it's might be to small? She's never showed signs of irritation or discomfort before and I've been riding her in this bit for about 2 years. She plays with it once in a while- like she's 'chewing' on it but her ears are always up and there are no signs of sores.
How can I check this and , if need be, measure my horse for a bit that fits better?