It’s tempting to just guess and hope for the best, but wasted space is wasted money, and not enough space is a problem you really don’t want to face. The only way to figure out what size is the one for your stuff is to have a crystal clear idea of what exactly is going into the unit (trust us—you don’t want the packing process to be the deciding factor).
Once you know what you’re storing, you can use this handy calculator to come up with a pretty good estimate of how many cubic feet you’ll need.