I hope I'm getting right what you are asking, to get the change in diameter, you need to set the Size control to Pen Pressure (in Shape Dynamics tab). However, that only works if you have a tablet with pen pressure.

Acolet is totally right about the brush shape! The other window with the different brush types is under that second tab at the top of your windowโ€”where it says brush settings/brushes. :smile: