My dad and me built mine. 14ftx10ft. Poured concrete to a depth of 12-18″ on top of a DPC.
While the concrete was wet 2x4s laid flat around the edges with long bolts/screws into the concrete.
When dry those 2x4s served as the base to build a frame from with 2 by 4s. The interior height is 8 feet to use 8×4 plywood/fibreboard sheets as the interior lining. The outer cladding is tongue and groove planks. There is insulation between outer and inner skins.
My dad made up oak window frames. 4 windows about 18″ wide x 2 ft high with frosted glass. Small enough and placed high enough it would be very difficult to enter the shed.
The door is a solid core fire door with 5 lever mortice. Better security than my house door.
The roof is coated steel sheets. Basically flat but with a slight slope to shed water. Maintainance free for 15 years.
Other than brushing the outide with wood treatment every couple of years and replacing one window sill I think ther shed will be goopd for another 15 or 20 years.
Size – It’s 14×10 feet and joins onto a lockup garage with a door through the common wall. Still too small 🙁
I couldn’t go bigger with the ground layout I have. I’d suggest always going bigger than you think you need – if you have the space.
Mine currently stores 7 bikes. Beer and brewing kit. All my tools. Loads of books. It has a kitchen counter the full 14 ft length of one wall which served as a cyber cafe for my kids and their friends for several years. Has a couch and a couple of chairs for the kids when they have friends round.
Sheds? Can’t beat them.