If it were me, I would  stand there with the washer wand  and simulate washing the biggest machine you've got then kick a spot on the ground. Get out a tape measure, measure from the center of the machine to your "kick mark" on the ground, then add 4-5 feet onto it.  (machinery always gets bigger thru the years) one way slope is ok but personally I'd opt for a center slope and run a "french drain" set up with a grate in the middle. True some mud would run into the drain but you could flush if necessary. Rest could be scraped off by loader.  Or, slope to one end then put in 'french drain' running to area away from any equipment traffic.

Some 'laundry projects' get pretty involved trying to clean,, and doesn't take long to start your very own 'fishing pond'.....