Unless you have a specific type of aquatic animal you want to keep, the general rule is try to go 'as big as you can'. in regards to aquarium size.
The reason is that the more 'water' the more 'stable' the water 'conditions'. Small aquariums change very quickly with the slightest 'variation' in water quality whereas 
larger aquariums tend to 'adapt'.  

For example; a 15 litre aquarium will very quickly become the same temperature as it's surroundings, whereas a 700 Litre aquarium filled with 'colder' water from a tap at 14 degrees will stay cooler for a longer period of time