If you haven't seen a reef tank below 10 gallons you haven't been looking very hard. Salt water reefs have been dominated the past 8 years or so with nano/pico reefs.
Plenty of tanks to choose from a quick trip to Petsmart or Petco and pick up an aquarium and an Aquaclear 110 and off to a good start. There is also a lot of small builders or tanks that will build to your specs if you want. Many of the readily available Fluval Spec tanks are used for reefs also. Many of the readily available stuff will need lighting upgrades but easy enough as LED systems for these size tanks are plentiful.