Fastening Crown Molding to Walls - WOODWEB- what size nail to use for crown molding ,I use 2" nails, but 1-3/4" should work provided you are nailing into wood. I have hung literally miles of ultra light 5-1/4" crown over the last 10 or 12 years using 16's. I only use the nail gun to quickly shoot the crown in place and in the corners where hand nailingCrown Moulding Installation Tips | The Royal Wood Shop1/10/2018· First of all determine the size, width and length of the crown molding you are going to install. If your crown molding is on the thicker side then you will need a strong support while installing it. A nailer of needle gauge length 15 or 16 is perfect for the molding.