Do I Need to re-inforce my shed base?

Buy the standard reinforcing mesh you can get from most builders merchants. Unless you are going to park heavy machinery or vehicles on the base, or load shelves with equally heavy items, the smallest sheets they have in stock will be more than adequate for your needs. Anything longer/wider will also be thicker and probably more than four times the price and less than twice the size. You are better off with more sheets of the thinner. Use bolt croppers, which you can hire from tool hire shops quite cheaply, to cut the sheets, of which you may need three for that job, to size. The edges of the steel can be up to 150mm (about 6 inches) away from the edges of the concrete - that is, the concrete is wider/longer than the steel. Do not have the steel sticking out of the concrete. Lay the steel sheets side by side or, if you have nowhere to get rid of any offcuts, simply overlap them. For the amount of force or stresses a shed floor will get, no need to be too fussy about how you lay them, just make sure no edges poke out above or to the sides of the concrete and that you don't have more than 150mm of concrete without any steel in it. Closing Tip: If the sheets available are the standard 3.6M length (x 2M wide) lay the longest side across the 12' width of the concrete and they should fit with just a few inches (less than 6) of free concrete at either end. Does your shed base really need reinforcing? Not necessarily. But the steel will prevent hairline cracks appearing in the concrete, which would eventually make it crumble and break away, particularly around the edges.