My ideas on all this:
1. You could possily try to enlarge them by transforming, but that would probably pixelate them, which is no good. Other than that I can't think of anything.
2. If you want to change that specific shape of the brush, I'd select the shape of the brush, create a path and trnsform that path and save it as a new brush.
3. Change the angle in the brush pallette in the brush tip shape tab. That should work then.
4. I just use several brushes along the edges, erase again some with other brushes, use more brushes, smudge, erase etc. until I get a look I like. Or I use photographs of all sorts of things to create grungy brushes. I keep taking pics of dirt and old wooden park benches nobody wants to sit on anymore

and use them later to create brushes. Those pics are perfect for that sort of grungy stuff.
HTH