Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
They do make pond liner that big, but if the area has a tendency to keep itself topped off anyway (even if just seasonally) then look into bentonite. It is a clay sort of material that will help the natural soil to be more resistant to draining. Not 100% impervious, but in a situation like this it might be the answer.
As to what to do with that much dirt, only dig it halfway, and use the dirt to build up the sides?