As far as engine size, mine is a modified 650. So anything larger than that will fulfill all your wants and needs.
I mentioned the Can Am only because they now offer a 1000 cc engine. MHO is that it is overrated and definitely unproven. Kawasaki, Yamaha, and even Suzuki seem to get the same job done dependably for less$ and with 7 to 800 cc engines.
As far as transmissions go, you're going to end up with some sort of CVT as that is all that's offered in the big bores to my knowledge other than maybe Honda. All manufacturers seem to give their drive systems some sort of name in order to differentiate them from the competition, but in truth the principal is all the same.
You mention pulling a trailer. Any of the big bores will pull much more trailer than you have any business pulling. The problem is not going but stopping. It's hard to whoa up a 6,000# trailer with a 700# quad but it can be done if you're careful enough