are small so that they slip between endothelial cells of capillaries to fight infection in the surrounding tissues

None of the above is related to plasma cells.

have a great deal of rough endoplasmic reticulum reflecting the fact that they secrete a tremendous amount of protein (antibody)

are large so that they can envelope their prey by phagocytosis

have a great deal of rough endoplasmic reticulum to dispose of ingested pathogens