I used to use a small 50 watt heater in my 25 litre fry tank. The problem with small tanks is the heaters are often too long though but if you shop around you should be able to find one that will fit it. Some can be fitted on a diagonal which allows more space. Some have to kept vertical though so it's always best to read the manufacturor instructions before you buy.
Most usual aquarium heaters can be placed diagonally (or horizontally). The problem with placing one vertically is that the heater warms the water, which rises, and the thermostat just sees the newly-warmed water, and therefore cuts off too early, before the warmed water has had a chance to circulate.