I just published the latest version (3.5) of the Particle Physics Simulator, with a fancy new feature that allows for different-size particles to be shot into the simulation. You'll notice a frame with a size-changing particle when you hold your touch down for a shoot. The size of that particle at the moment you take the shot is the size of the particle being created. Easy, ain't it?
Additionally, there are a number of other improvements and bug fixes, go discover them yourself!