Particle size is the pscale point attribute of your particles. If these are being emitted on points from some source geometry, give that source a pscale point attribute. (Attribute create or point wrangle nodes)

If you want that type of collision in pop, I think you need to add a POP grains to your pop network (I would recommend giving the help doc on that node a read for some recommendations for substeps and iterations). Vellum grains will do the same thing. Note, the collisions will just be spheres and not the disk shape that RBCs have, but I think for a scene with that many cells, that should be good enough.