No, I don't mean you Sierra's, I mean garden sheds!!

We are now preparing the base for our shed, which I had planned to be around 8ft x 12ft with a double door, but may opt for 7ft x 13ft. I want to be able to keep the Gas BBQ in there and would like to put the patio table and chairs in there over the winter (although that's not essential). It's hard to percieve whether I am along the right line's size-wise.

Also, I have dug to about 150-200mm deep and plan to build a wooden frame, which I will half-fill with hardcore, then concrete. Does that sound right? Once the concrete is hard, do you remove the wood or leave it in place? Once we have done the base, I planned to dig the surrounding grass out, and back-fill it with Gravel to make the area around the shed low-maintenance. Someone also said that you should make the concrete base very slightly smaller than the shed, so that water can run off the shed and onto the surrounding grass/gravel instead of running onto the concrete base and gathering under the shed.

Any help is appreciated!!