To continue with the ship editor discussion, since that is where this thread drifted too...
The best way to look at this for purposes of creating an editor like this is that the fundamental basis of ship design are the break points in mass that allow for the mounting of the next bigger size of engines, and how many engines a ship has. It is important that larger engines are more efficient/miniaturized and that, for example, 2 small engines take up more of the "available space for this mass" than a single medium engine.
So, to keep this simple, you might have three engine sizes... exactly like you already have. Using SFB as an example since this is so well-established there, imagine that small engines provide 15 thrust, mediums are 30, and large engines provide 45 thrust. The higher the mass, the more space there is for engines, weapons, crew quarters, bowling alleys (thank you Dr. Sorensen...), etc. In addition to this, certain break points in mass allow for the mounting of larger engines.
The size and number of engines a ship has is the sole factor in its classification. So, using SFB as an example... a Destroyer has two small engines. A heavy cruiser has 2 medium engines. A heavy battlecruiser has 3 medium engines. A Dreadnought has 3 large engines. And on and on. It isn't the size and number of engines that determine class, it's that those class names describe what modern gamers would call the "min/max" for how squeezing that many engines of that size onto a ship while still having the space available for weapons (and anything else represented in the game) that needs to be on the ship.
This is the beginning, this is the foundation of the ship design side of the kind of editor that you are wanting to have in Star Control.