The seeding particles are an inherently critical component of the PIV system.  Depending on the fluid under investigation, the particles must be able to match the fluid properties reasonably well.  Otherwise they will not follow the flow satisfactorily enough for the PIV analysis to be considered accurate.  Ideal particles will have the same density as the fluid system being used, and are spherical (these particles are called microspheres).  While the actual particle choice is dependent on the nature of the fluid, generally for macro PIV investigations they are glass beads, polystyrene, polyethylene, aluminum flakes or oil droplets (if the fluid under investigation is a gas).  Refractive index for the seeding particles should be different from the fluid which they are seeding, so that the laser sheet incident on the fluid flow will reflect off of the particles and be scattered towards the camera.
The particles are typically of a diameter in the order of 10 to 100 micrometers.  As for sizing, the particles should be small enough so that response time of the particles to the motion of the fluid is reasonably short to accurately follow the flow, yet large enough to scatter a significant quantity of the incident laser light.  For some experiments involving combustion, seeding particle size may be smaller, in the order of 1 micrometer, to avoid the quenching effect that the inert particles may have on flames.  Due to the small size of the particles, the particles motion is dominated by Stokes drag and settling or rising effects.  In a model where particles are modeled as spherical (microspheres) at a very low Reynolds number, the ability of the particles to follow the fluids flow is inversely proportional to the difference in density between the particles and the fluid, and also inversely proportional to the square of their diameter.  The scattered light from the particles is dominated by Mie scattering and so is also proportional to the square of the particles diameters.  Thus the particle size needs to be balanced to scatter enough light to accurately visualize all particles within the laser sheet plane, but small enough to accurately follow the flow.
The seeding mechanism needs to also be designed so as to seed the flow to a sufficient degree without overly disturbing the flow.