Landscape artist Mike Shubic can make you a humongous guitar like the 12-foot-long beaut seen here. This particular style starts at $6500 plus shipping. From his eBay listing:

This is an acoustic style guitar sculpture that can be made in any size, finish or style, and, you can actually strum it. The strings are piano wire in various sizes and the piece can be tuned (to some degree).

Link (Thanks, Michael-Anne Rauback!)