Comparing to size:
* As small as a pebble
* As small as a grain of sand
* As small as a pinprick
* As small as a child's hand
* As small as a mouse
Comparing to other concepts:
* As small as a whisper
* As small as a flicker of light
* As small as a drop in the ocean
* As small as a speck of dust
* As small as a fleeting moment
The best simile for you will depend on the context and what you want to emphasize about the small size.