I think you are going to be left with one of 2 choices here.
1.) Open CD, Right click, open "settings", go to "background", and adjust the "Normal Size" slider.
Make sure CD is in another area, as you can see the effect in real time.
Also from the backgrounds menu you can choose other backgrounds, but I think you already know this.
Also under Dock Shape in your settings you can adjust Icon separation, how many icons per circle and separation between circle layers
2.) Make your own background. All backgrounds are in PNG format, and there is no apparent size restrictions.
Backgrounds are very easy to make with either Gimp or Photoshop.
Hope this helps