Some features in RF5 are not available directly in RFRK 2.0, one of them is the possibility to animate the radius or splash, those which are included in the sequence tab:
To animate the particles attributes you have to do it manually, I mean using a simple mel line. First go to the frame you want to animate and set the value, then write in the Script Editor:
Where files is the first emitter in the tab list and radius is the attribute to set the key.
This video shows four particles with an animation in both Radius and Plygone Size attributes.
Note: Thanks to Mihnea Balta for the tip.