As a food source for copepods, amphipods and other life a refugium can be much smaller than the main tank to give benefit. In your case 10 gallons would probably suit. However, as an effective nutrient removal system you may as well get a 20 gallon....IMO this would allow a large colony of your chosen algae if thats the way you are going. If you have space for a 30 or 40 gallon even better. Remember you are increasing the total water volume thus increasing stability within the system.
Lee