Reproduction steps:
1. Open attached project
2. Open and play the "Scene" scene
3. Notice that all particles are the same size (check "Capture 5.3.PNG")
4. Open the animation windows and notice that "Particle System.Initial Module.start Size.scalar" property is missing (check "Missing.png")

Actual result: Particle System initial module properties cannot be animated.

Expected result: Particle System initial module properties should be animatable (check "Capture 5.2.PNG")

Regression introduced in: 5.3