Don't count on the premium sites being larger than the preferred sites. Disney didn't change the actual length and width of the sites, nor did they widen loop roads. It appears that all they did was replace the coquina pads that used to be there with concrete and widen the aprons. Some of the largest sites in the campground are the 1600-1900 full hookup loops. There are most definitely some premium sites that'll be tight for you. We were assigned to a site in the premium 1400 loop on our last trip that was smaller than any site I've ever had at the Fort. While we could get into the site without any problem, we couldn't fit both our 45' motorhome and our car onto the site - the motorhome took up nearly the entire length of the site and literally came within a couple feet of the road, and there was barely room next to it for the picnic table - forget a car. I had to call the front desk and ask to be switched to a different site. They then tried to put us on a site in Little Bear (the interior 1400 loop), where we'd have trouble getting around the loop, and that's premium as well. We've actually stayed in the 400 loop twice in the past year, and we liked our sites (although not all sites in a loop are similarly sized) and the proximity to the settlement. 500 looked nice as well. 700, which is also close to the settlement, was under construction when we were last there, so I haven't seen it completed.
Just be sure to tell them the size of your equipment when you make your reservation (or call back to have it added to your reservation) and tell them that fitting comfortably is your first priority (if that's the case). Then, if they put you in a site not to your liking, have the front desk number programmed into your cell phone and call them to get it switched before you waste your time backing in.
We don't usually take our dogs to the Fort, but when we went in March/April, we took our nearly 5 month old puppy along because the people watching our other dogs at home weren't comfortable with a puppy. We used the Disney kennels when we spent full days in the parks (both the one at FW and the one at Epcot), and we were very happy with them. It's a nice option to have if you're not sure how your dog will do alone in the camper for an extended period.