I used to workout on hardwoods that were very slick, so I put my yoga stickymat under the step. That kept it from sliding around and I had not much excess sticking out from under the step.
Another option that I've heard from some people is to buy the interlocking alphabet letters like the kids have, once you lock them all together, you get a nice size workout area (since you don't want to slide around either!)
HTH,
Tricia