I can resize the brush easily with the mouse in Lightroom but even in CS6 it's very old hat to have to use the keyboard very annoying when you export from LR4 to PS and lose this intuitive option. Why oh why is it not in PS6.
There are many ways to resize your brush in Photoshop.
Right-click on your canvas to display the Brush Picker. Drag the Size slider.
Hold down Option+Control+Drag your mouse left/right (Mac), Alt+Right mouse button drag.
[ = Decrease brush size.
] Increase brush size.
Not good enough though Howard. Both Lightroom & Aperture allow changing brush size directly with mouse which is way more intuitive and user friendly than Photoshop, if lightroom does why doesn't Photoshop.
It's a bit more difficult with Photoshop, as the scroll wheel can be used for zooming as well (General Preferences). In Lightroom, the scroll wheel doesn't really have much purpose, other than to adjust the brush size when the Adjustment Brush is active.
With your Magic Mouse, hold down Option+Control and drag left/right.
Do you have a Brush Tool active when attempting the shortcut? Make sure you're pressing the mouse button down as well.
Hold Option+Control(Not Command), then Click+drag your mouse left or right.
For the record....
The reason ctrl+opt+drag is inferior to mouse wheel is because it causes you to move your mouse pointer. Not just the mouse, but also the pointer. For those of us who, when we draw, are making lots of minor adjustments in size as we draw, this is not an insignificant issue. With the current implementation, you drag to resize which is fine (though odd) but when you let go the cursor suddenly snaps to the new location away from where you were pointing when you just wanted to adjust the brush size.
The obvious way to fix this is just to allow for different options in the preferences. Some will want mouse wheel to zoom (when no other key is held), others would prefer it the other way around.