1. Silt: Silt is the smallest of these particles, typically between 0.0039 and 0.0625 millimeters in diameter.

2. Sand: Sand particles are larger than silt, ranging from 0.0625 to 2 millimeters.

3. Pebbles: Pebbles are larger than sand, ranging from 2 to 64 millimeters.

4. Boulders: Boulders are the largest of these, typically larger than 256 millimeters (10 inches) in diameter.

So the answer is silt is the smallest.