I know the formula in rietveld refinement for finding particle size but unable to know the D2ST
This probably would have been easier for many powder diffractionists with a clear explanations what D2ST stands for; most of us have little knowledge of arenology. I had to look up this term and a number of definitions in a book, "Aeolian Sand and Sand Dunes". I assume d_st is Stokes' diameter of a free falling particle, which is related to two other diameters as d_st^2 = d_V^3 / d_d.
The volume diameter d_V in this is the diameter of a sphere with the same volume as the particle. The Scherrer equation typically employed in Rietveld analysis gives the volume-weighted diameter, also called d_V, which is the average length of the chord of the sample, that is one cuts the particle into parallel columns and averages their length. The exact relation between the two depends on the shape of the particle. This Scherrer shape factor is 0.89 for spherical to 0.94 for cubic particles. Usually, this is set to 0.9 for particles of unknown size. It might already be included in the software analysis.
Be aware that X-ray diffraction gives the size of the coherent scattering domains. The particle size is a upper limit for this, but it might be much lower, if the particle consists of multiple domains.
Next, the drag diameter d_d depends on viscosity of the immersing fluid. But in the region, where Stokes' equation is valid, d_d = d_s should hold, with d_s the surface diameter. If one assumes a sphere as the particle size, surface and volume diameter become the same, and d_st = d_V
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