Marine fish actually need a lot more space than their freshwater counterparts. This often is due to the size that many species grow to, but can also be down to territorial instincts and general aggressive behaviour towards other fish. Obviously if you put two aggressive fish into a very small tank the consequences could be nasty.
The minimum recommended size when starting a marine tank is at 80 litres or 20 gallons.
Smaller systems can be kept but the number of fish you can keep becomes very limited and the smaller aquarium can be prone to greater fluctuations in water parameters. An 80 litre aquarium will give you enough water volume and potential filtration capability to look after a small collection of smaller fish including members of the clownfish, goby and damsel families. You would also be able to accommodate a few invertebrates and selected corals. A bigger tank would always be better and the larger the tank is, then the larger the range and size of livestock, you will be able to keep.