Q: Why is my Adjustment Brush so small?

The brush size is measured in pixels.

Are you working on an unusually (for you) large Image?

But first, make sure that your "Brush Range" is set to "All".  This setting is found on the HUD Action Menu (click the "gear" icon).

There are some things you can try along the way, but you're headed towards deleting your preferences as described on the Aperture trouble-shooting page.  This usually resolves issues relating to a mis-behaving UI.  You could go straight there, or try:

- rebooting

- seeing if the problem exists with other brushes

- seeing if the problem exists when you log on using a different User Account

- seeing if the problem exists in a newly created Library.

Each of the above helps pinpoint the cause of the problem.

Added:

Forgot to ask:  what pixel size is shown in the value slider for size?

Message was edited by: Kirby Krieger