I'm not completely sure on how long or what roughness of sand paper to use, as I've never had to do it (I clip nails) and I've only heard this method through the grape vine.
If it were me though, and I had to come to some conclusion for my own rats then I would probably use medium-grain sandpaper and use it one the wheel one day on and two days off. I would be inspecting feet after each day that the sandpaper was on though to make sure nothing like scratched up skin was happening. The good thing is that rats are usually very smart and won't continue doing something if it hurts them. You will need a lot of sandpaper though, because I can guarentee that they will chew on it. It might even become one of their favorite pasttimes. That's another reason why I would only use it one day at a time, because chewing on sandpaper probably isn't the healthiest thing to do.
You could double-up on methods (use the bricks too) so that you achieve maximum nail-shortening.
I wish they made those little "soft claw" things for rats like they do for cats and dogs. That would be very convienient.
If you do decide to clip nails, I reccomend two people (one to hold and one to clip) and only clip off the clear, thin, sharp end. Don't clip off the entire nail. If you make a mistake and it bleeds, use cornstarch or cornmeal (put the bleeding toe into it and it should clot most immediately).
You might find that wrapping the rattie in a thin towel while clipping nails makes it a lot easier. I use baby nail clippers (human baby that is) with my rats. Don't use the scissor kind, they are too hard to control.
Good luck.
-----
*Sara*
along with:
3 ball pythons
1 burmese python
1 black-lined plated lizard
3 leopard geckos
lots'a ratties 
2 gerbils
fishies
2 dogs &
1 kitty