A: I would use carriage bolts. Either stainless steel or zinc. Both are available at Home Depot. Depending on the metal for your trailer. You probably need 2-1/2 inch or longer. You can use 1/4 inch diameter if you have several rails or 5/16 or 3/8 if not.
A: If you are using PG MAX Stainless Steel Screws there are 68 screws per pound, one pound installs approximately 19 square feet, five pounds installs approximately 97 square feet, and ten pounds should install just under 200 feet. This is when driven 2 fasteners per joist, 16" on center, on 6" wide decking; allow for some waste and loss during installation when estimating required quantities.
A: 8.5mm-9.0mm (.335”-.354”)