I agree with sublight, the one on the right is a better icon.
Its still very early in the development of the UI so I'm not too sure exactly what button icons we'll need, but for now I could use:
Ship Icons for System Map (Different for each Classes/Role).
Some icons for the different planet Types (gas Giant, Terrestrial, Rocky, Ice, Asteroid, Comet, etc)
Some icons for the Star systems on a galaxy map (could do different icons for single, binary, etc. Systems maybe?)
Arrow Buttons for the UI.
Zoom buttons for the UI.
Icons for Economics, System Map, Galaxy Map, class Design, Ships Screen and System Information.
As for the icon size, the Winforms toolbars use 16x16 pixels by default. We can resize the buttons but I'm thinking no larger then 24x24 for those. On the OpenGL size I'm currently using 8x8 coloured circles for the planets/stars/etc, however this could be increased to 16x16 or even up to 32x32. Just keep in mind that the OpenGL stuff gets scaled based on zoom so these icons will need to scale well.
Also could you make the backgrounds transparent instead of black? They'll mesh better with the rest of the UI the way (especially in the OpenGL stuff, black backgrounds behind every icon just looks wrong
).
I'm still not sure on the style myself, the retro style is okay but after seeing the below Chrome Experiment the other day I have been re-thinking our current UI approach a lot:
http://www.chromeexperiments.com/detail/100000-stars/It'd be a killer combined system/Galaxy map, but retro icons wont mesh well