This is a 24 foot long picket fence. When you have a very lightweight fence you can put your 4x4 posts 6 foot apart. This is non-load bearing so 6 foot would be fine. Now we have used pressure treated posts here. At your local lumber store you can go and purchase 8 foot pressure treated 4x4's, cut them in half and you will have exactly 4 - 4 foot tall, 2 feet down into the ground and 2 feet above for your frame. Now on this picket fence the frame, the bottom and the top are pressure treated 2x4's. Now since this is a 24 foot long fence we will need 6 - 8 foot 2x4's. Now when you measure for your top frame piece make sure that you measure from the center of your 4x4 post to the center of your next 4x4 post. This will give your frame here a nice anchor and support. Now on the bottom portion of the fence, on the bottom frame you measure from the inside of your 4x4 post to the inside of your next 4x4 post. The other materials that you need to install your picket fence would be your exterior screws, don't forget your drill, your backup battery, a plumb line, and a framing square.