yeah, there should be a scale button on the toolbar. I believe it's next to the rotate button, but not sure (don't feel like double checking right now). Just select the entire prefab, then hit the scale button and then drag until the desired size is reached. This may or may not cause some parts of the prefab to get distorted. It depends on how complicated the prefab is I think. I haven't had any real problems with it yet though.
Ep. 3 Jedi Starfighter