As long as you have dirt or sand to mix it with, you really can't put too much. In a bathing area that size, I'd probably mix in a cup or two....
I punched holes in an old coffee creamer tub so that I can squirt/shake DE out like talcum powder. It comes in handy to squirt a little on pudding poos or really watery poos that you're trying to clean up - dries them right up...lol Works great on dropping boards, if you use those...