OK, I have CS, so my
Brushes palette is under
Windows > Brushes. The hot key is F5. From there I can go to
Brush Presets or
Brush Tip Size and what you want is
Shape Dynamics, also mess with
Other Dynamics. Let me know if this works out for you.
Here is a picture, but this is Photoshop CS. What you want to look for is in the yellow. You also have to select the brush tool to access the palette.