Water Heater Size depends on many factors. So it is difficult to conclude the size without knowing your requirements. If we consider a very common case scenario of a 4-member family, then an instant water heater with 5 liter capacity is enough considering you are taking a bath using a bucket. Shower wastes a lot of water. So if you plan to take a shower, then you need a storage water heater of at least 10 liters capacity.