I have another comment about map size: Maybe you might also consider doing something to make the map "feel" larger subjectively, and that would be to rework engines, and reduce the speed you can design ships with.
Right now, in my opinion there are several problems with engines. First, engine size and cost don't scale (enough) with hull size. It's simply way more efficient to use big hulls than small ones, because for many small ships you pay way more for engines and life support, both in mass and in cost, than for a few large ones. It would be completely logical to expect engines and life support to be both larger and more expensive for a bigger hull (more mass, more personnel).
The second problem is engine stacking. Stacking linearly adds the speed of the components, while the reachable area per turn (i.e., amount of space one fleet can defend) grows as a square function. I would recommend you consider one of two solutions to this: Either, completely remove engine stacking, and just allow a single engine per ship (with increased movement value), or change the movement value of stacked engines to the square root, so the reachable area grows linearly.
In my recent experience, the AI doesn't really use engines all that well. If there was a single engine per ship rule (and consequently, reachable top speeds on the order of 10-12 per turn), AIs should be both much more competitive, and maps actually feel larger, because it takes longer to travel all the way through them. It really is true that ship speed matters, if your fleets are way faster than the AI, you can defeat even a stronger enemy with relative ease, since you can achieve local concentration of forces and your enemy cannot.
By the way, I thought your recent solution to sensor stacking spot on, congratulations