1. The two features that would allow you to enlarge the image are "Plot Area" and "Scale". Most likely you have Plot Area set to "Extents" which is the entire extents of the data that you have loaded in... (so if you have another layer that's a fab drawing with border the extents will be huge -- and not a good fit). I recommend turning on only the silkscreen layer and then use Zoom All (menu: View | Zoom | All). Then when you go to Plot Wizard make sure Plot Area is set to "Display" and Scale is "Fit to Page". This should get you better results.
Note: You can also set the Margin to 0 (located in Advanced Options).
2. To invert the silkscreen, I recommend using the command "View Backside" (go to menu: View | Backside. Then create your PDF for documentation.