how much does size (firebox) actually matter for inserts?

Go as big as you can and stay away from flushed insert. I heat roughly 1200 of 2x6 house constructed in 1994. My insert is right in the middle of the house in kitchen living dining room combo with a cathedral ceiling. It heats very well I can get high 70s no problem if temp are in the 20s when they are below I have to push it harder but still 72-74 is not a big deal for the stove. My problem is you have to run the fan, there is some radiant heat coming out of the huge glass but not enough to heat the house. Biggest problem is the shallow box. It's just almost impossible to use its 2.2 cf fire box to its full potential. Stove is beautifull no question. All that said I am strongly considering BK princess next year.

Congrats Trogdor. That's a beautiful insert! Cast iron cover plates, 2.5ft3, n/s loading! You have to post some pics when installed.

As applies to a lot of things. Two sayings come to mind. "Better to have it and not need it than need it and not have it." and " no replacement for displacement"