I'm looking into this.
First thing - the concrete will cost more than you think, and (wit prep) possibly as much as the rest of the build.
Second - no-one ever said "my shed is too big." (Actually not quite true, but the person I know who said that had $zillions and just bought a second set of tools to go at the 'other' end.)
Will you be using this throughout the year? If so, insulation (underfloor and elsewhere) is going to be important.
Are you living rurally? If so, you may be able to get this through as a rural building, with the slight relaxation in rules that goes with that.
There are many possible sources for a commercially supplied shed. Fieldays in Hamilton is just around the corner and there will be deals to be had there.