Alternatively, you can open the Brush Preset picker by right-clicking with any brush-type tool.
With the Master Diameter slider you can adjust the size of the current brush (it won't change the preset brush's properties).
To apply your changes and close the picker, either press [Enter] key twice or start painting. [Esc] key discards the changes.
To change the other settings, use the Brushes palette (pull down menu Window > Brushes). To save an edited brush as a new brush preset, click the icon on the Brush Preset picker.