Rain gardens can range in size from small, simple gardens for the average homeowner to large, complex bioretention gardens. Rain gardens absorb 30% more water than the equivalent area of a typical lawn, and the root systems of the plants and layers of porous soil provide even more capacity to filter pollutants. Redirecting a downspout to a rain garden will not only help reduce flooding and pollution, but will also attract birds and pollinating insects, adding to the biodiversity in your neighborhood.
The Winooski Natural Resource Conservation District created the Vermont Rain Garden Manual for properties in the Lake Champlain watershed. It includes information on how to determine size and plant species.