I have a 16×10 with an extra 1m deep rear veranda for outside storage. Should be about right for what you want as a minimum.
As I have said before I would build my next out of 200x47mm T&G dungwall. Will need a chainsaw to break through it. Agri store do it on offer for £15 (inc. VAT) for 4.7m or £20 normally or £28 for 6.1m – could make a nice 20×10 shed very easily from that.
Double skinned, insulated roof with steel box section on top. Find a source of cheap tempered glass for the windows (eg ikea shelves) and make a double wall window with glass on the outside with reflective film and polycarbonate on the inside (glass won’t scratch like PC).