I found they are generous, foot size wise. The regular width fit me fine in my old boots but I ordered the same size in new ones last year as they were a bargain and I couldnt get the zip past my ankle ooooops so hard to say.
I think the sizing is generous. I'm an 8 in ariats but a 7 in montain horse. And a regular calf when I had to just sneak up to a wide calf in toggi Calgary. I also have quite wide feet and my running trainers are a size 8.5 (ffs). So yeah. Mountain horse seems a generous for to me.
I think they depend on the model. My High Riders are roomy in the foot, my Firenzes less so. They're both fairly generous in the leg. I have very wide calves and struggle even with wide fit in some makes but my mountain horse boots are fine.