I have one on either side of my driveway gate. They are 12', I have them 4' in the ground and have two 12' cattle gates hung on them (24' opening) with an electric opener. The poles were here when I bought my property, I just found a use for them.
I also have a 48' X 50' bulding with 6" X6" posts. When I put up the building I found a local guy that had a contract to get the old power poles from the local utility Co. He had a sawmill and cut the poles square for me.
I don't know about your area but here the poles have a lot of takers, they might be hard to get. But to answer your question, they would make great fence posts.