Hello; I ran the numbers. 35cm x20cm x 20cm = 14000 cm cubed. That appears to convert to 3.698 gallons if I am correct. For such a small tank a 25 watt heater with a good thermostat should be enough.
On that point, in my experience the quality of the thermostat more often plays a role than the specific wattage. With a good working thermostat the heater will shut off within the desired temperature range. Too much heater wattage for a tank, especially with only one heater, has the potential to overheat the water quickly if the thermostat fails. I tend to shy away from higher wattage heaters anymore and use what may be considered closer to minimal. Two or more heaters of modest wattage reduce the potential of cooking the tank in a failure.
That said, much will depend on the environment. A tank in a room of a house where the ambient temp is kept within a few degrees of the desired tank temp will need only a modest heater to make up the difference. A tank in a garage or basement may need stronger heaters to keep up.
I like to run two different wattage heaters, say a 50 watt and a 100 watt. I try to set the 50 watt to come on first as my tanks are in a heated room of my house. The 100 watt is set a few degrees cooler and only comes on when the smaller heater cannot keeep up.
Two heaters also help when one fails as the other may keep the tank warm enough untill a replacement can be had for the broken one.