Within the thermodynamic Gibbs potential the theoretical consideration is given for possibility of forming a vortex line in superconducting particles of different shapes having the transverse size smaller than the penetration depth. The expressions are obtained for the threshold fields corresponding to forming and trapping a vortex during field cooling and zero-field cooling, demonstrating the dependence of these fields on particle radius. This agrees with the experiment performed in powder-like HTSC having different distributions over the particle sizes.