The particle size of clay soil is

Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates i.e. sand, silt and clay. These particles determine the properties like porosity, permeability, infiltration, shrink-swell rate, water holding capacity and susceptibility to erosion.
Sand is the largest particle of the soil whereas clay is the finest one. The particle size of clay soil is less than 0.002 mm.