I have no idea what sized nails I used. They are from Home Depot and in a bin where you buy them individually rather than in a box. These pictures will give you an idea of size. They are thick enough I don't get any deflection. The smaller nails on my cargo rack would bend if I really cranked down on them as they are smaller nails.
http://i36.photobucket.com/albums/e1...k/DSC07718.jpg
http://i36.photobucket.com/albums/e1...k/DSC07721.jpg
FWIW I have less clearance between the bottom of the rack and my roof and it's quiet. I did notice when I put the front piece on my rack I hear the wind going through that section when the sunroof is propped open. Nothing from the back section and nothing touches when the rack is loaded up.