You, too, can reflect on your own situation, work with it, and develop your own personal rain garden. Estimate the area from which your garden will get rain. This is how big your garden should be. Remember, though, that different parts of your roof drain to different downspouts. Bannerman notes that rain gardens for single family homes will typically range from 150 to 400 square feet in breadth.
Rain gardens vary in depth, as well.
The norm is from three to six inches. If you have poor infiltration, make your depression shallow to reduce the water volume that gets trapped there. Contrarily, if your soils suck water up, make your garden deeper to increase its storage capacity. Sands' wetland garden reaches one to two feet deep. Whatever the depth of your garden, be sure to keep the bottom fairly level so that water spreads out.