If you are thinking of using your roof gutters as your rain garden, you should not put this garden too far from your house. It is best to install it at least 10 feet away from your house. Roots and utilities underground can also affect the installation of your rain garden.
You will need to dig a depression if your yard does not have it. You can do it using an excavator or manually. There is no need to make it very deep. You can just dig a basin of not deeper than 5 inches. The basin should have slightly slope slides and even bottom.
To make a berm for holding excess rainwater, you can put some dirt that is enough for the plants to absorb the water.
Though a small rain garden is not bad, the ideal size of your rain garden is at least 150 sq feet. This is enough to have various plants that can achieve your desired landscape. You can opt to have rounded shapes for your rain garden such as teardrop or oval.