Just use the random expression in size and angle, excerpt from manual:
random(x)
Generate a random float from 0 to x, not including x. E.g. random(4) can generate 0, 2.5, 3.29293, but not 4. Use floor(random(4)) to generate just the whole numbers 0, 1, 2, 3.
random(a, b)
Generate a random float between a and b, including a but not including b.
So for the angle you can put random(360), for the size - if you want a proportional size you would do something like:
Set Sprite width to random(16,32)
Set height to Sprite.width