Mine was 17 X 12 must admit a bit extra would have been useful,
An empty shed looks gi-normas, till you get the benches all round and the shelving
the centre bench for delicate jobs,
Pillar drill, belt sander, grinder, then Two lathes. er! an extention needed. but it was cosy and fully insulated a greenhouse heater completed the job.
so I think your 20X12 is a good size, ask when you order it I got an extra foot at the same price,