There are a number of different propane tank sizes out there, and the size you get will be dictated by a couple of major factors. Namely, you'll need to consider exactly what you'll be using the propane tank for and how much propane you think you'll be using. For example, there are a number of smaller propane tanks that you can get that work perfectly well with bar-b-ques. This is because they are mobile, and hold enough propane to have a decent lifespan with basic use. However, getting this same size of propane tank for your hot water heater may not be such a good idea.