Ours is something like 7' x 10' and I wish it was bigger because your crap always expands to utilise the available space! An 8" foundation for a shed may be excessive. We built ours on a 4" slab poured directly onto compacted dirt and it hasn't moved.

The slab should certainly be very slightly smaller than the shed - about an inch off each side is sufficient.

Spend the time getting your wooden frame dead level (allowing a very slight run-off may be an idea, but not hugely necessary) and solid before pouring the concrete and it is dead easy to slide a straight edge across the top to get a decent finish.

If you want mains power now or maybe in the future, be sure to dig out and insert a suitable conduit that will emerge just above the level of the concrete wherever you would want to bring the power into the shed.

Similarly, you may want to think about the rainwater that will come off your roof - its a fair area. If you plan to include a gutter and downpipe, either provide a hook-up to the drains or - if you are into gardening - you could plan somewhere for a water butt to gather the water.