I would try to stretch it out a little. 24x20.
Or 20 x 28 or more. Rectangles are generally more usable than squares in the building world.
20x20 will be the largest...
Could do 16 x 20... or 16 x 16, but "...a 20’ square (400 sq. ft.) happens to be that magic number that’s minimum size for bank financing for some banks and maximum size (even with the loft) to qualify as a tiny home with relaxed code requirements under Virginia building code Appendix Q. Many uses, including primary mini home, accessory dwelling unit, detached office, family room wing, hunting / fishing lodge, etc., etc. "
