VEVOR PEX Heat Transfer Plates, 300 pcs Box Radiant Heat Transfer Plates, 4 ft Aluminum PEX Heat Transfer Plates, 1/2 inch Heat Transfer Plates Designed for PEX Tubing

Yes you are right, when you are looking at the sight there is a place to choose over on the right side of the computer screen you are looking at and you can pick the length you need. I would not buy 2 foot lengths it would drive you insane when doing infloor heat for your second floor

Yes you can, would like to get more information on your project. I’ve done multiple floor types with in floor heat.

I used a staple gun with great success. Make sure to overlap plates a bit for better transfer.

The aluminum type is 6063.

While the product description doesn't mention anything about outdoor use, customers mostly use it for indoor installations. Many mention its effectiveness in improving heat transfer and lowering water temperatures.