I'd been struggling to find out how to do this using touch controls since I had put W8 on the RM Slate and thankfully have found a video showing me how to do this. Using a mouse is pretty easy, you simply right-click the app in Metro and you get the option bar appear at the bottom of the screen giving you the smaller/larger, uninstall and unpin options. Using a touch interface however is a different matter. What you have to do here is tap and hold the app icon and drag it down and back up slightly to make the option menu reveal itself, and TBH, it's a pretty hit and miss affair. I had been pressing and holding hoping for the equivelent action.
Anyway I've attached the clip below (courtesy of groovypost.com) to help you all out.