6kw is quite a lot even for a large room.
Especially as you say it’s well insulated.
Obviously you have to be getting every bit of heat out of it to achieve 6kw.
I heat my entire house off a 14kw stove (inc rads and hot water) and it’s mostly just tickling along after the first few hours of use.
Give me some dimensions inc windows and outside walls and an idea of how old and how well insulated and I’ll tell you what size you need. remember a bigger stove just eats more and more fuel. Your better off running the one you’ve got full rather than half using a massive one.
20 foot ceilings though are going to kill you as noted above as you’ll struggle to heat the top half let alone the bottom in a sensible timescale.