[Suicide Junkie]

You may or may not want to use this:

Approximate
I = M * R^2

Approximate R = V^1/3, where V ~ M
I = M^(5/3)
Assume thrust is proportional to mass, for ships are usually the same speed.
The maximum torque available is Force * R, which becomes M * M^(1/3) = M (4/3)

Torque over angular momentum gives acceleration of M^(-1/3)
Since we're dealing with constant speed instead of constant acceleration, square that to get M^(-2/3)

So, unless I did something wrong,
Turn rate should be proportional to 1/ shipsize^0.6666