It simply depends on two issue:

1) Availability of particles (all sizes are available there)

2) If all sizes are available, then based on shear velocity (which is a function of the velocity), and settling velocity of that size of particle and turbulent Schmidt number you find the Rouse number (dimensionless number)

if Ro>5 that particle does not move

if Ro (2.5 to 5) it moves and jump as bed load

if Ro (0.7 to 2.5) it is become suspended

if Ro is less than 0.7 then it is always suspended (known as wash load)

Well,

adding in brief, Suspended solids size depends on immerse and buoyancy ability of water.

Regards

do field study and Laboratory analysis to find a trust answer

Thank you very much Kaveh Zamani , Bayan Hussien , Mohammed mostafa Mohammed Slim and M.Hassan Nasermoaddeli