Replying to A few questions about the setup im working on +K

So I have a drop in thats a little over four feet tall, and im going to start working on a flat rail. This is going to be a winter-only setup. How long do you think I could build the rail and keep speed? there isnt much slope to my yard. I was hoping 10-12 feet, but the rail is going to be metal so im not sure. As well i was wondering what types of metals slide best. I've heard copper and steel are both good, and can get both for around the same price. Any advantages or disadvantages of each? thanks guys