it will be nice to have option to scale the particle size based on emitting face size(similar to dupliface with scale).it will be useful for replacing leafs from ivy generator or tree leaves from Ngplant with custom leaves/mesh.
You could do this in Blender with excessive, excessive scripting, but overall this sounds like a job for Houdini.
Random size is the only alternative I could think of to this, but yeah, the particle system leaves much to be desired at this point.