You need much more specialist equipment. This is an area where I have considerable experience (over a decade) as we sprayed emulsion commercially .

First you need a good spray gun & compressor, a garden spray is inadequate, then you need good fume extraction and an airline respirator so thet the air you breathe is from outside the darkroom, then you need a means to develop, fix, wash etc which will require a lot of water and drainage. The compressor needs to be isolated from the darkroom so it feeds clean air to the airline respirator which is fed by the same airline as the spray gun.

We used an emulsion which had been designed for spraying, we made our own, and regularly worked around the size you're looking at as well as larger.

Ian