While your brush is active you can adjust some simple settings, such as Size, Focal Shift (the softness at the edge of your brush) and more. To access the options you can use the sliders above your viewport, but for a faster workflow get used to holding down Space.
ZBrush provides various methods of accessing options, so you can tailor the interface to your preferences. You can even choose your preferred brush by pressing Space, which helps to speed up your workflow. These simple sliders make even the standard brush a versatile tool.