[Master Belisarius]

You can design ships with a determined size (for example, missile boats with the Destroyer hull).
But the problem is that in the end game or games with all the tech from the start, these ships are useless and AI doesn't build the enough number of these small ships to hurt a fleet of dreadnaught.

Also, the AI can have more than one ship design, for the same class of ship.
For example, you can specify one attack ship with only the Destroyer hull, and other attack ship with a hull from scout to dreadnaught.
The problem here, is that you can not tell to the AI, exactly how many ships do you want to build for each class.