On the Options bar, open the Brush Preset picker and double-click a preset brush to choose your brush size and style.

As default, the Brush Preset picker in Photoshop 7 presents the brushed in the "Stroke Thumbnail" viewing mode. The brush strokes allow us to judge about the brush dynamics. However, you can choose "thumbnail" or "text " view (movie).

If you use a pressure-sensitive digitizing tablet in Photoshop 7 you can choose a dynamic brush (brush, dynamically changing shape, opacity and / or color).

In Photoshop 6 you can turn brush dynamics on / off by using the gadget at the right end of the Options bar.