Imagine a ship comprised of nothing but an engine, shield, computer, weapon (phaser or photon torpedo or some combination thereof) and other ancillary systems (commo, sensors, etc).
I do. And what I see is one of the automated sections of the Prometheus
Anything smaller would not have an engine, because combat-credible engines are big things. It might not even have a weapon, because we haven't seen evidence that ship-killing weapons could be made particularly compact.
Take a look at today's drones.
Yup - aircraft that are just as large and heavy as their crewed counterparts, and thrice as expensive. Because they have to be. Anything smaller would be unable to carry the weapons and sensors or mount the engine.
Which is completely separate from the issue of whether or not they would have a cause to launch another war. Right and wrong don't enter the picture at all - especially not right and wrong as defined by the enemy.