Normal brush increase / decrease with a keystroke setup of [ and ] works fine.
Opening liquify filter and try to increase or decrease the brush doesn't work at all. Seems the keystrokes in the liquify filter are different (and unknown?!) to increase & decrease the brush size but WHY?
After clicking in the Brush Size field the North/South keyboard arrows should change brush size, same for square brackets, its slider should allow you to change size and keying in a number should change Brush size. Don't any of those work?
Setting up the Wacom Intuos 5 and Photoshop CC 2014 to...
Brush decrease: #
Brush increase: NumPad *
Keyboard language: German
Works with normal brushes and in the Liquify filter.