When I do this I use "every tick" and "system create object"
create a cloud off the left edge of the screen.. at Y random(300) for the X and Y size you will enter "random(10,100) or what ever you like, this will create random sizes.
and then a "every tic" to move the cloud "forward" and random(10) or what ever speed you wish.
this will give you, random size, random speed and random positions
hope this helps