How to Increase brush size with swipes?

Hello
I'd like to setup BTT to change the brush size in Photoshop, with upward and downward swipes. But I can't manage to do so. I can already change the size, having configured custom keys to do so in Photoshop, the Z and the A keys. Is there a manual somewhere to explain how to do so, or did any of you guys manage to do something similar?
Cheers