I put an A-Frame together last year as a grow-out area for my chicks, before I finished cleaning up the old coop.
First time doing it, I made mine 16' long, and around 5' high at the center. That was a mistake. It's too difficult to move around. If I installed wheels on one end I could move it enough with a dolly, but still not very easy to move.
If I did it again, I'd make one shorter in height and length. I think they're a fine concept, if you get set up so you don't ever need to go into the things. Something low enough that doors on the top/sides would allow you to reach to the ground and halfway across the width. That way you could reach everywhere without having to either crawl or hunch through the inside of the thing.
Maybe 8' long, and 3' high?
I chose to do an A-Frame rather than a hoop-coop, because working with the materials for an A-Frame came more naturally to me. If you've got the space for a higher structure, I think I like the concept of the hoop-coop better. It's lighter, and more easy to access.
On the other hand, an A-Frame seems easier to store out of the way when not in use.