(just throwing ideas out there)

Maybe you could add a penalty to larger ships and a bonus to small ships--something like {[SQUAREROOT](hull/3)} - X

Where X is a predefined number--I'm thinking 2 or 3. The result is the number to add (or subtract) to it's current speed. If we take X to be 2, then a hull 75 ship would end up adding 3 to its current speed, making it much more likely it can only barrel straight ahead, but a hull 27 ship would add 1, and a hull 12 would add nothing, a hull 6 would subtract 1 and a hull 3 would subtract 2, making it much more likely to be able to come about or reverse course.

That would mean a large ship might be massive engines, but it's going to turn like a swan on a frozen lake, whereas smaller ships can turn like a hummingbird.

Of course it is all a matter of taste, but I like to differentiate between speed and agility to a greater degree than the Thrust Rating feels like to me. As I said in my first post when I was looking at historical ships there were VERY large cruisers that were faster than some destroyers, so I was trying to come up with a way that the cruiser could never turn with the smaller ship, even if it could outrun or chase it down with ease. There is also a lot of precedent in science fiction where there is a gulf between pure acceleration and the ability to turn. The simplest idea I had was to slightly alter the REVERSE COURSE orders only as this would alter the turning radius of different ships.  Ships Rated A can complete the Reverse Course as described in the rules. Ships rated B must move one hex between each hexside turn after the first, ships rated C must move one hex before each hexside turn (this means it must move one hex forward before its first course change), ships rated D must move 1 hex before the first turn and 2 hexes before another turn, Ships rated F may not make a REVERSE COURSE movement.
A Thrust 5 Destroyer with A Maneuverability moving at 3 could turn, turn, turn, move ahead while a Thrust 6 Armored Cruiser with Maneuverability D moving at 3 would have to move ahead 1 hex, turn, move ahead 2 hexes, then turn.
If I am playing this right it would look like this:
[attachment=1]Setup.PNG[/attachment]
[attachment=0]Reverse.PNG[/attachment]
If the big cruiser wants to make a "tighter" turn it had to slow down and use COME ABOUT orders rather than REVERSE COURSE. It will take several game turns to come around 180 degrees but she would still have the same ability to accelerate at a very high level.
Anyway that was my thought on it. I could see where coupling it to the existing size categories would work.
Erik