particle size

So, I am working on the attached composition and I want a way to adjust particle size over time. I want the particle source to spit out particles of varied sizes. All I've been able to figure out so far is how to make all of the particles on the screen grow and shrink in unison by changing stuff in particle render: structure, but that's not what I want. I have also tried changing the size, mass, and area on the generated object itself, but this seems to have no effect. Ideas?