I'm not aware of sheets larger than that.
If you plan on modifying/using a large cutting board, check your sink size first. Most dual sinks won't handle much more than 9 x 11; consider using a large-base, low-height "Tupperware" (for the lack of a better term) container instead of a sink.
You might want to get several cutting boards, to accommodate various grits you plan on using.
Use contact cement to affix the sandpaper; when the paper loses its "grit", peel it off, clean off the contact cement with acetone (in a well ventilated, spark free environment), and put new sandpaper on.
I'm curious why you need larger sizes of sandpaper...what scale are you building?