I can't help you with controls on your tablet since I use an Intuos 2 and don't use even those buttons.
Here are a couple of ways to change brush size using the keyboard (I hold the stylus in my left hand and use the keyboard with my left hand):
Hold down the Ctrl+Alt keys (Windows) or Command+Option keys (Mac). Your cursor will change to crosshairs. Click and begin to drag the cursor and you'll see a dashed line circle indicating the brush size with the crosshairs cursor at the edge of the circle. Drag your cursor out to make the brush size larger or in to make it smaller.
Click the [ key to make the brush size smaller, click the ] key to make it larger. Hold down either of these keys to decrease or increase brush size continuously until you let go.
Using the Size slider and/or Size number field on the Property Bar (I do this with my stylus and right hand for the arrow keys, though I could just as easily use my left hand for the arrow keys.):
Size slider - Click and drag the Size slider left to decrease brush size or right to increase brush size.
Size number field - Highlight the existing number field and click the up arrow key to increase brush size in percents. Click the down arrow key to decrease brush size in percents. (As I recall, it's something like 5%.) If you hold down the up arrow key or down arrow key, the brush size will be increased or decreased continuously until you let go.