This is a suggest guide that I often see on the net.
1.5 litres of water for each centimetre of fish length (1 gallon per inch).
30 square centimetres of surface area per centimetre of fish length (12 square inches per inch).
Of course there are many factors that can influence the above guide like.
filtration, aeration, surface agitation, water change frequency etc. Not to mention the types of fish that are being kept.