[Suicide Junkie]

There was once proposed a system that might do what you want...

(Exact exponents are arbitrary)
1) Make build costs rise roughly as the Square of the hull size.
2) Ship Effectiveness will be linear with size, no matter whether you use QNP or standard propulsion. (The smallest ships under standard fall a bit below the linear, but we can ignore that)
3) Reduce maintenance costs so they rise only as the square root of the hull size.

As such, a large ship 4x bigger would cost 16x as much time and money to build, while being 4x as powerful (same strength per KT), and costing only 2x as much in maintenance.

In emergencies, you could build small ships quite quickly, but if you have the time and resources, large ships will offer more bang for the buck in the long term.

During times of conflict, you will lose some of your big ships, the biggest of which will be effectively irreplacable.