Take a look at our 2008 board here:
http://www.chiefdelphi.com/media/photos/29506?
We support the plastic sheets along their long dimension with aluminum angle. They are otherwise very flexible and wouldn't do well alone if left unsupported across long spans.
We cut everything from 12x24" sheets that run about $20 each, whether they be the 1/4" ABS from years past or the 1/8" delrin we used this season.
We use these plastics and in these sizes primarily because our lab has a laser cutter with 12x24" of travel. Once the layout is designed in CAD and exported to the machine, we can have a board cut in about ten minutes. For folks who're working with other tools, other options may work better for them.