Information about a 25 watt stealth marineland heater for aquariums?

I bought a 25 watt stealth heater. I was wondering if it would work in a two gallon tank. I was also wondering how submerged it needs to be. It says that it has a safety shut off when it is out of the water; however, there is a piece towards the top where the dial is that is thicker than the rest of it. I was wondering if that part can be left out of the water.