You should be fine with 6 600*600 concrete slabs, just space them out a little to ensure no overhang. Lay them on some leveled sand.
Might I suggest the gentleman consider the solid Steel shed? Rot-proof, scrote-proof, doubles as a high volume percussion section when hit with large bits of wood.
If you go with a wooden shed, give the standard felt a miss and treat yourself to a 2 layer commercial felt roof that should have a 15 - 20 year life. We've just done that with our 8x12ft shed and it's so much drier already, it'll be the same stuff they put on garages & extensions. We paid £42 a square metre. If you can go bigger, 6x4ft fills up pretty quickly...
I bought a plastic shed from keter 8x11, not very secure I suppose in that you can kick it in if you really wanted being plastic but I would say wood is easy to break also if you really want to, compared to my old wood sheds, no wildlife has burrowed in, no spiders, no damp, it looks as good now as it did 6 years ago, it has a light plastic channel in the Apex of the roof to let light in and being sand coloured it is very bright in there, I’d buy again, looks very nice, doesn't age.
