The staple length you need for an upholstery project (or any project, really) depends on the thickness of the material you are stapling, and what kind of base material you are stapling into. Obviously, the thicker the material, the longer the staple, but the base material matters also because you want the staple to go all the way into the base without any parts sticking up. This means that for harder base materials like oak, MDF, and plywood, you should use shorter staples, and for softer base materials like pine, cedar, or foam, you can use longer staples. The general rule of thumb is that you want at least 3/16" of the staple to go all the way into the wood. So figure out how thick your material is and add about 3/16". For upholstery projects, I almost always use 5/16" staples. If you're not sure about the length you need, buy the length you think you need AND the next shortest size. If the staple stands away from the surface (i.e. the crown of the staple isn't fully flat against your fabric/plywood) when you try to staple something, the staples are too long—go down a size and your staples should go in nice and flat!