Agree with Colin, fish need enough horizontal swimming space to easily get to full speed if they want to, otherwise, problems with obesity and muscle loss, along with the associated long term health problems, can occur.
The majority of solitary fish are recommended at least 6 times thier maximum adult length on average, with many of the more active species doing better in longer tanks than this.
For schooling fish, it is generally a good idea to at least double this length to 12 times the maximum adult length for schooling fish, once again with this as a minimum and longer tanks being better.
A couple of important notes on this:
Be sure to get a second opinion on maximum fish size, some shops underestimate this, especially for larger species.
In general, Aquariums under 15 gallons, or about 57 liters, unless they are specifically stocked with the smallest available species, tend to reach the stocking limit faster than many would like for a general community aquarium.
In general Aquariums under ten gallons are more difficult than larger tanks to keep, being inherently more unstable, and having much less margin for mistakes that may not cause any effect at all in a larger tank.