Here are the detrital rock grain sizes, from smallest to largest:

1. Clay: < 1/256 mm (less than 0.004 mm)

2. Silt: 1/256 - 1/16 mm (0.004 - 0.0625 mm)

3. Sand: 1/16 - 2 mm (0.0625 - 2 mm)

4. Gravel: > 2 mm

Note: Gravel is further subdivided into:

* Pebbles: 2 - 64 mm

* Cobbles: 64 - 256 mm

* Boulders: > 256 mm

This classification is known as the Wentworth Scale, which is a widely used system for describing the grain size of clastic sediments.