This is arguably the most important question to ask yourself when purchasing storage. You need to consider the space you have available, this will help you to initially start narrowing down the size. Then you are going to want to think about what you want to store and how much of it there is likely to be.
For simple items like towels or outdoor pillows, a small 50 gallons box will suffice. Medium deck boxes hold up to 100 gallons and are great for garden tools and supplies. If you need to store pool maintenance equipment, the large ones go up to 150, and the extra-large 150+ gallon boxes are able to house everything you keep outside.