I have heard that extra large sheets such as you are suggesting were developed by using a paint roller dipped in the developer and the developer allowed to work until there was no more activity. The same was then done with stop bath and the fixer. Needless to say this required a large room with a stone floor and facilities to hose the print down afterwards. It seems a monumental task and not one I would care to undertake.

Alternatively, you could make large shallow dishes out of thin plywood (you would not need to be a qualified carpenter) and line it with sheets of polythene (waterproof tape any seems). That would not be too expensive and eminently more practical than the roller method.