The sedimentary rock with the smallest grain size is claystone.

Here's why:

* Grain Size: Claystone is composed of particles smaller than 0.0039 mm (1/256 mm) in diameter. This is extremely fine, making it feel almost like powder.

* Formation: Claystone forms from the accumulation and compaction of clay minerals. These minerals are often derived from the weathering and erosion of other rocks, and they are transported by water or wind.

While other sedimentary rocks like shale can also contain clay, claystone is specifically defined by its predominantly clay-sized particles.