[@Maxwell] you can pick an existing one or create one of your own. And yes, a bigger engine holds more power, but each chassis size tells you what size engine they have, so there will be no 20 core engine. 

Generally, 
Light = 3 core 
Medium = 4 core 
Heavy = 6 core 

If you notice, the next-gen models have generally more powerful engines than when compared to current gen machines, which gives them an advantage, which is sort of the point- but most of my pre-made units are purposefully unoptimized, in order to balance that out. All player made machines will be current gen machines.