All brushes have a variable size that can be changed.

You can change the brush size in several ways:

Using the Size slider in the tool options. Pencil, Paintbrush, Eraser, Airbrush, Clone, Heal, Perspective Clone, Blur/Sharpen and Dodge/Burn tools have a slider to vary the brush size.
By using the default shortcut keys for changing a tool's size:
- Decrease size by 1: [
- Increase size by 1: ]
- Decrease size by 10: {
- Increase size by 10: }
By using the default mouse scrollwheel actions for changing a tool's size:
- Decrease size by 1: Ctrl+Alt+Scrollwheel Down
- Increase size by 1: Ctrl+Alt+Scrollwheel Up
By configuring the mouse wheel behavior in the Input Controllers preferences:
1. In the main window of GIMP, click on Edit → Preferences.
2. In the left column of the new window, select Input Devices → Input Controllers.
3. The Additional Input Controllers preferences are displayed with two columns: Available Controllers and Active Controllers.
  
  In the column Active Controllers, double-click the Main Mouse Wheel button.
4. The Configure Input Controller dialog opens.
  
  In the left column Event, select the entry Scroll Up.
5. Click the Edit event button (at the bottom middle of the list).
6. The Select Controller Event Action dialog opens.
  
  In the Action column, expand the tools item.
7. In the left column Action, select Tool's Size: Increase Relative, then click the OK button.
8. Now the Scroll Up event is associated with the action tools-size-increase-percent.
9. Close the window.
10. With the same method, configure Scroll Down to be associated with Tool's Size: Decrease Relative.
11. Click the OK button of the main window of Preferences.
After these somewhat long explanations, you can use your mouse wheel to vary brush size. For example, choose the pencil tool with the Circle brush. Set the pointer in the image window, and use the mouse wheel in both directions to see the Circle shrinking or stretching.
By configuring the behavior of the Up and Down arrow keys of the keyboard:

The method is similar to that of the mouse wheel. The only differences are:
- In the column Active Controllers, double-click Main Keyboard.
- In the column Event, edit the entry Cursor Up for the first key, and Cursor Down for the second key.
- Then, use the Up and Down arrow keys of the keyboard. The result is the same as with the mouse wheel.