How do you measure a horses mouth for a bit?

I have a 14.2hh pony and she looks like she has a rather small mouth and her bit looks a little bit big in it. Like you have a space at each side. I didnt know if you where sposed to have a gap or if there is sposed to be no space. Atm she has a 5" bit. Has anyone got any idea about how to measure her mouth and how it should fit?

Atm she is in a fulmer snaffle.

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