The size of a pond hose consists of two factors: the length and the diameter. It is the length that determines how far you can carry the water using the hose. If the plant or whatever else you want to water is far away from the hose, then naturally you want a longer hose. Because it is almost impossible to find a hose that has the exact length that you need, you should be going for the one that is longer than the minimum length that will work for you. You can figure out how much diameter you want. If you are going to use the hose for watering a plant, a standard hose will do. On the other hand, if you are going to fill up a large pool using that hose, then the diameter of the hose should be significantly more.