Ruby is not the programming language of SketchUp. SketchUp is written in a compiled, closed down language and we do not get to see what makes SketchUp work. They have opened small portals into SketchUp. That is where Ruby comes in. Ruby is allowed to interface with the core SketchUp program. But it can only do what they have designed it do it. They have not allowed us access to change the toolbar icon possible sizes, therefore we can not do it.
But the workaround is to make your own icons on the screen like some of us have played with. Or to make a Web Dialog with your own icons.
Besides, I bet no one would want you to change their icon size for them. People have very strong feelings about how their UI is set up. And no one would want a script to come in and change things outside the realm of that script (like global icon size). Good luck with it,
Chris