How can I change the icon size in Lauchpad?
I have found no way to do that, nor have I heard of a way.  MacOSXHints is the place to watch for this kind of thing.

I have created folders in effect by dropping similar apps onto each other.  All QuickTime apps are together.  All Maintenance Apps are together, as are media viewing apps, etc.  I also move the least used apps to page 2

This reduces the number of pages for Launchpad and makes apps more accessible for me.  My most-used apps are still in my Dock so LP is rarely invoked.

This doesn't give you the solution you wanted, but may help as workaround.