Get yourself familar with this:
For “Growing” Particles, you need to use the “Initial Size” and “Size Over Life” Modules.
The Initial Size determines the Initial Size of the Particles in relation to the Emitter Lifetime.
The “Size over Life” Module is a multiplier in relation to the Particle Lifetime.
The “Initial” keyword is somewhat misleading in conjunction with Size over Life. I would rename “Initial” with “Base”, since it is a constant value for the Particles and is multiplied with Size over Life.