A good size for a compost bin is between 5 to 7 feet (1.5 to 2m) wide and 3 to 4 feet high (0.9-1.2m). However, in a small garden, 3 by 3 feet (0.9 by 0.9m) is a usable size.

(I bought two 12 gallon plastic storage bins to make mine.) A lid for the interior bin will keep flies from getting in. Screen door patches and superglue will keep your worms in and bugs out.