paintbrush tool allows size based on pressure and results in a single central spline that can be adjusted. not sure how that translates into after effects but sound slike it does what you need within photoshop.
just choose the paint brush. double clikc on one of the 'round' brushes in the brushes pallette and you can choose from the drop down menu to have size based on pressure, then choose the amount of points you want it to vary from the stated size.