Brush size hotkeys

I'm using a Wacom drawing tablet. How do I increase/decrease brush sizes? Shift + scroll is not an option, since I don't have scroll on the tablet pen. Numpad +/- are only zooming the image. Sometimes I've been able to use ] and [ brackets as in Photoshop, and it ACCIDENTALY worked. But usually it isn't working. I have no idea why.
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