The best shape depends on your space, as you want to work with gravity rather than against it. Most rain gardens are designed to come from a downspout and move downward. People also design them alongside a driveway or ditch for a longer, more narrow design shape. You want to maximize infiltration. Guide the water to meander or pool through the garden so that it filters before draining to an overflow drain. Also ensure you’ve called your local authority and mapped out any utility lines on the property when designing your shape.