It depends on what you're storing, but for a lot of people, an 8×10 is a great starting point. It gives you 80 square feet, which is enough room for a riding mower, yard tools, a few bins of seasonal gear, and still have space to move around. For 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.
