Eelie, I believe the number keys only control the size of the spray can tool, 1-5 for round brush sizes and 6-0 for square brushes. In order to control the size and color of the marking pen, after selecting the marking pen, right mouse click anywhere on screen and hold, at the top of the screen you'll see the colors and a slide bar to control the size. But I really don't think this is his problem, I'm on a Windows platform so I really can't help in that regard.