how do you scale the units to the more realistic sizes?, I can't seem to find it in the options

Look in the units XML file and find the Scale_Factor, just change it up or down.

Larger numbers increase, and lower decrease the size.

 

Note: the star destroyers for instance have a value of 1.0, but fighters have a value of 0.7, etc...

So they don't all have the same value. The Venator (marked as Jedi_Cruiser) is roughly the same size as the Victory, but has a scale factor value of 1.2, while the Victory has a scalefactor of just 1.

 

Note 2: If you are going to change it, remember to change the ships' death clone value aswell so the blown up ship won't appear smaller (or larger depending which way you scaled them) as it explodes.

Yes Petroglyph made this mistake on the Falcon, if you notice when it blows up it doubles in size.