In After Effects, resizing a Paint Brush is fairly straightforward. Control-drag (Windows) or Command-drag (Mac OS) the brush in the Layer panel to adjust Size, and release the key and continue to drag to adjust Hardness. For more, see Forgotten AE Paint shortcuts here and Brushes and the Brushes panel in AE Help.
In Photoshop, shortcuts for paint tools can be a bit more involved, especially if you irregularly move between platforms. You can use the bracket keys  to change Brush Size and add the Shift key to the mix to change the Brush Hardness. In Photoshop CS4, with a Paint Tool selected, you can get the Brushes preset picker with Control-click (Mac) or Right-click (Win).
Even better, to change Brush Size directly hold the Control + Option keys (Mac) or Right-click + Alt (Win) while clicking and dragging the mouse. To drag-resize Brush softness just add Command or Shift; on the Mac you use Control + Option + Command while drag-clicking, while on Windows it's Shift + Right-click + Alt.
A red overlay temporarily previews the Size and Hardness of the Brush; this color can be changed in Preferences > Cursors > Brush Preview. Julieanne Kost has covered other Brush shortcuts.