Hmmm. I went to both Ham Hill and Cadbury Castle last weekend. One of the info boards at the sites said it was the largest hill fort in the country, but I can't remember which one it was at!
Yes Ham Hill definately counts as a large one, I'd place that as third largest, but my knowledge ain't bang up to date.
Here's my stuff on Rolston Scar:
http://www.brigantesnation.com/SiteResearch/Iron%20Age/RoulstonScar/roulstonscar.htm