What effect does a larger or smaller diamter have?

Disclaimer: the following is based primarily on my own deranged reasoning

I believe the diameter to mostly influence inertia. A smaller diameter presents a lower moment of inertia (rev quicker, less torque) while a larger diameter presents a greater moment of inertia (rev slower, more torque). The speed at which a given converter stalls is a function of input power and the angle/shape of the stator elements as they interact with the thing** that I can't think of the name of at present.

Then again, I should've been asleep an hour ago.

** turbine, perhaps