Here's our Magnum Zero Clearance. The firebox is about 1.5 cu ft and it'll easily take 22" logs. It'll do a decent job of keeping our 3,000 sq ft house at 68° even with outside temps around -10°. However, once the temps drop below that, we really have to stay on top of keeping it fed and stoked.
Output air temp (lower right corner of upper grill)