Hi, When Emitting, the basic values such as size, are set while the particles emit, so you will see the keys affect the result only for new particles, e.g. if you have a grid that was already emitted, you will NOT see it affecting, while if you have a Box emitter that keeps on emitting , new particles should be affected by these keys.
The same goes for the most basic particle properties , these are set on birth, and you can control the overlie properties using the various Over Life graphs, and the other tools , such as Turbulence, Transform etc’ 

Thanks for the answer. I\’d love to see this become animatable for existing particles in future a update. I did find a way to animate the scale of existing particles with fields. Hopefully that will help someone else in the future.