Just some food for thought. I am currently building a 3x3x6 with 6 being the height. I figure that I can get about anything I want to coat in it easy enough. A friend across the road is into bikes and I have a couple of my own. Mine are vintage Honda 750's. I assume your talking about Harley's and the like. Either way, wouldn't 6 feet tall and 3 x3 be a good size for that type of stuff? Tall would be easier to get into and wouldn't take up so much room as one laying down.
I have pictures posted of mine, as it is, at this point and will be adding some more in the next day or so. Take a look at mine for size and such.
http://s80.photobucket.com/albums/j1...Powder%20Coat/