The problem with swivel jacks is that they all are crank types, and that will slow you down unnecessarily. For stabilizers, you really want a drop leg type of jack, or a swing out jack, with about 1" height increments. Then you can set and retract them instantly.
As far as height, I normally leave 1" to 1.5" of gap (hand/knuckle thickness) between my blocks and the trailer frame, and that's enough to accommodate the load and let the blocks slide out freely after loading. The blocks are at the very back of the trailer, so they only carry the load as the tractor is driving up the ramps and over the rear. Once I get forward enough to have proper tongue weight and level the trailer, there is once again space between the blocks and trailer.