Two dealers--two different suggestions on BTUs

Is there a lot of glass in this addition? If so, much of your radiant heat from a stove will be lost there perhaps allowing you to up the size a bit.

Will the stove sit right across from the opening to the other part of the house? If so, i'd consider a stove with a blower to push warm air directly into that other part. Also, a ceiling fan would be wise to have running as well (you decide if you want it up or down, i've read many arguments for and against either!)

Perhaps your best bet would be a catalytic stove. If you buy a decent size cat stove, and you find it is too large, those are meant to be throttled right down and still give you a clean burn. That might be the best of both worlds for you.

pen

I'd be looking at ~2 cu ft stoves which is where the Shelburne fits in. The F3CB is a willing little feller, but only has a 1 cu ft firebox. If the stove is going to be used for 24/7 heating it will need more capacity or you will be feeding the stove frequently. The F500 is the 2 cu ft Jotul. If the place is exceptionally insulated the F400 (Castine ~1.5cu ft) may be a good compromise in the Jotul line, especially if the flue is going straight up in the interior of the house. It will give about a 6hr burn with good hardwood.

If non cat stove is the choice then a mid-sized soapstone or perhaps cast clad stove should work out. They give off less radiant heat and convect more. That is probably what you are looking for. Hearthstone makes mid-sized soapstones like the Phoenix or the Heritage. You can get a similar convective effect from a cast-iron clad stove like the PE Alderlea T5 or Quadrafire Cumberland Gap. Both are 2 cu ft stoves.

If cat stove, then a Woodstock stove or perhaps a BlazeKing Princess might work out.

Regardless of choice remember that you are not going to be seeing steady xxxx btus out of the stove. They heat more in cycles from refill to refill. If you want less heat, add less wood.