I was asked a few months ago to make some large letters for the Tampa Bay TEDx Youth event that happened December 2nd. I made the biggest styrofoam letters on my homemade CNC machine that I could - which put the letters at about 3 feet high. Here's a photo gallery from the TEDx event and the letters can be seen in more than a few of them. Thanks to Terri Willingham from Learning is For Everyone (and numerous other organizations) for giving me the opportunity to help out with the event (even in the smallish scale that I did) :)
Jamie