The plans do not include any plans for a floor. There are too many options for floors to choose from and some of them may depend on local building codes.
Concrete Slab
Boxed & Joists with plywood on top.
Boxed & Joists with decking material
Grass
Gravel
On an existing deck.
Making Changes to accommodate different sizes
to make it wider or narrower, change the length of the roof lumber to be longer or shorter
to make it shorter/taller, change the length of the wall 2x4s to be shorter/Longer
to make it deeper/shallower, change the number of frames IE: buy more kits to make it longer, use less frames to make it shorter (or just space the frames closer together). Currently they are designed to shed specs of 24" OC. Normal house is standardized at 16" OC.