I absolutly love PS of Sweden bridles, I bought one a couple months ago and they sent me a hole measuring guide to know the exact size for my mare. The disadvantage is that PS of Sweden bridles are a bit expensive.
I bought my horse an ovation bridle a couple months ago and I had ordered the oversized because I have a big warmblood. I got it and realized it was waaayyy too big, so I downsized to a horse. I tired that and it fit well, although it almost is too small for my horse. It really depends on the bridle because each bridle will fit differently. I definitely recommend Ovation and Pessoa bridles. I'm not sure how to measure, but just have to use your guessing skills.