Dictated by the size of the existing hole. Probably 1/8 inch dia. If the hole is oversized 5/32. Use only all Aluminum rivit including the mandrel [nail]. Grip length is important to get a clean break off inside the compressed rivit. If the rivit is to long part of the mandrel may stick out after break off, ugly and sharp. Usually 1/8" grip lenght for two sheets of aluminum only, longer if its where the skin is attached to hull frame. You can get all alum rivits in various colors for interior work.