Re: Concrete Pad (slab)
A perfectly flat slab won't hold water, it just won't shed it as quickly as a sloped slab. For the size you're talking about, don't worry about it, just set your forms level. When you screed it off, DO NOT over-screed it, the more times you drag a screed across it, the more concrete you'll remove, and the more likely you'll create a divot that will hold water. Also, make sure you crown your screed (bow up ).
You should also keep the top of the slab higher than the surrounding dirt, the amount will be dependent upon the slope of the surrounding grade and other landscape details.
Since you're going to put a shed over the slab, you have to worry even less about water issues/drainage, if you're really concerned, you can size the slab to the shed and use Z-flashing around the perimeter to prevent water intrusion, but again, for a generator, not an issue. Also, make sure this shed is well ventilated and that the generator is exhausted outside of the shed, both of these things will prevent toxic gas build-up within the shed, whether the generator is running or not.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!