I don't have any personal experience with the Sienna, but I recommend first figuring out exactly what yours is rated to tow. A good place to start is the towing guides on Trailerlife.com or similar. If you don't have detailed info from Toyota already.
http://www.trailerlife.com/images/di...wGuide0801.pdf
Once you figure out what your vehicle is rated to tow then you have an idea of what trailers are within it's towing capabilities.