Harman XL

Well the installation of the Nashua stove is complete, Tempetures have gotten down below freezing a few times and I can get all night burns. Really very happy with it. I am a little concerened about whether it will be large enough to meet my heating needs and burn times when temps get down to 20 or below. The firebox holds about 15 lbs and thats loaded and heaping. I have seen a few Harmon Mark 1 stoves used and reasonable and also an older Harman XL that looks good. Do any of you know about firebox sizes on these particular stoves. The Harmon XL looks like a great deal but it is about a 4 hr round trip. Any help appreciated.
Jim