The first thing I'd ask is how your game world's sensors actually work.
Detecting mass is pretty sophisticated, but reading the results of an energy emission is pretty simple. Having a reactor expel a plume of energy or broadcast just generic EM noise of an equivalent ship shouldn't be too hard. I would suggest it be limited by time and obviously not fool the naked eye. I'd suggest them being one-shot and have a limited duration...say 10 minutes.
As for volume, I'd use the following rules of thumb for sizes
Battleship: Diesel train engine
Crusier:box truck or large van
frigate: large refrigerator
Fighter: 2-3 large portable ice chests