Amber, the closet you can get to that size (in a 300gsm weight) is 51" wide. Lanaquarelle offers both their CP and HP in roll format 51"x11 yards. Click on this link for more information: http://www.legionpaper.com/lanaquarelle
You might also consider using a sheet of Rising Museum Board, 60" x 104", 4-ply, White. It is 100% cotton, and will provide you with a 5 x 8.7 ft surface.
David Carruthers of Papeterie Saint-Armand, in Quebec, Canada, custom-makes 5 ft. 10 in. x 10 foot (and 5 foot x 8 foot) sheets of 300 lb. watercolor paper and will ship to the USA. It's gorgeous stuff--expensive, but worth it. I think they're the largest you can get anywhere short of making them yourself somehow. He's quite nice to deal with. Here's the firm's url: http://www.st-armand.com/.
I'm looking for 6 x 8 ft sheet of 300 lb watercolor paper. Price is not an issue. Is there a company that will create these custom for me?
Thanks!