Well I don't know about particleforge, but you can increase the size of the exhaust effects by accessing the appropriate effect file in the Particle folder and changing the values for ParticleWidth & ParticleHieght lines.

Oops, I forgot to mention that the exhaust effect file's you most likely need are in the Original Sins Particle folder. You can copy any relevant file's you need out of it and paste them into the Particle folder of your mod. Just make a new folder if you don't have 1 already, and name it "Particle".