It depends on what you're storing, but for a lot of people, an 8×10 is a great starting point. It's an 80-square-foot footprint, and for yard tools, a few bins of seasonal gear, and basic backyard storage, it gets the job done.
Where it gets tight is if you've got bigger plans. Workbenches, woodworking tools, multiple large items, or anything you want to actually work inside of. If that sounds like you, stepping up to a 10×12 or bigger is worth it. We always tell people: think about what you need today, and then think about what you'll wish you had room for a year from now.
