The quick way is to use the b hotkey. Just press the b hot key, then mouse down in the canvas and drag an interactive marching ants line to specify the new maximum brush size, then mouse up.
The b hotkey resizing tries to be smart about adjusting any parameters the user might expect to change when their perception of the brush size changes. So things like path length or texture scaling will adjust as well.
If you use option b, then only the brush source size (H and V size) will be adjusted. Not any additional parameters associated with the paint, like texture scaling, path lengths, etc.
The maximum brush size is specified in the Brush Source control panel. You can set the horizontal and vertical dimensions of the source brush.
The actual brush size used for drawing is also a function of the controls on the Brush Modulation control panel in conjunction with the maximum size specified in the Brush Source control panel.
The Brush Size Range dual slider specifies the range of possible brush sizes as a percent of the source brush size. When a size modulator is specified the brush size modulates between the minimum range size and the maximum range size. If no modulator is specified then the brush is the Source Brush size.
The maximum path length is adjusted in the Path Shape control panel.
Here's a more detailed discussion of changing the brush size, including a short discussion of a virtual brush.
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There's also a tip available on Adjusting the Brush Size.