Steps to reproduce:
1. Open attached project and its scene "test"
2. Select "Particle System" in the hierarchy and set its "Start Size" value to bigger or smaller than 1
3. Run the scene and press "A" to observe the change of particle system's parameters which have been specified by calling Emit(ParticleSystem.EmitParams emitParams, int count) in the "PartTest" script that is attached to the "Particle System"
4. Observe that new particle system's parameter "Start Size" resets to ~1 if it is not specified in the Emit(ParticleSystem.EmitParams emitParams, int count) script parameters

Actual behavior:
The "Start Size" of the new particle system resets to ~1

Expected behavior:
The "Start Size" of the new particle system should keep the value that is specified in the inspector if it is not specified in the script

Regression appeared in 5.4 version