For planet-sized monsters I suggest going beyond the stats and adding both the terrain and the actual monster. Kind of like a living dungeon.You may have to adjust that - the largest size established by this project is 2000x2000 feet, see Epic Damage by Size table. You can get around that, but you need to be careful with it. The question to ask, what does the larger size get me? I developed the colossal (titanic) parts to address titanic creatures, but even that doesn't really makes sense for a planet sized monster. It becomes more terrain than monster, which some doo, but I am not personally fond of.
Also, just being of titanic size doesn't mean your effects and attacks are all equally titanic. They way I worded it the cloud increases to grow and grow and can get to the size you mention (with enough time), but it is not instantaneous. Personally I find the idea that a cloud can cover millions of miles in 6 seconds a bit ridiculous. That is essentially a super nova type of effect, which maybe what you want I guess. I just find it less interesting.
Personally, when it comes to these Elder Evils I take the size to be a myth and use what makes sense. So Leviathan wrapping the world of Atropus being the size of a planet are just mortal legends. In reality they are indeed titanic, but not so grand as the myths of mortals. I take the general stance with all aspects if myth, legend, and lore for deities, archfiends, and other epic monsters.