Is it possible to animate hair particle (object) size? I am trying to create an animation where several instances of an object seem to grow out of another mesh, and slightly move (rotate) then disappear (scale back to 1 again).

In the dumbed-down version of this example, I would have the cube grow out of the icosphere in random sizes, rotate slightly/move, then shrink back to size 0.

I cannot seem to animate / keyframe the Particle Size data box.



After keying it on frame 1, it doesn't let me key a second size on frame 50.

I couldn't find help online or figure out why this wouldn't let me do it. Even if I click on "No" or "Keyed", and apply loc/rot/scale, it doesn't let me change the value. I actually have to go and delete the keyframe to modify the value.

 

Can this be done?
I'm getting the same result as you, changing the value after keyframing the Size property reverts to the previous keyframed value; it may be a bug or a limitation of the current system.

As a workaround, you can animate the value through the Graph Editor.

Just keyframe it regularly as you already did, then open the Graph Editor, select the object and move the curve vertex corresponding to the desired keyframe up or down according to the effect you wish to achieve.