Why do we recommend going larger? It's easy . . . most of us are guys and you know how guys are . . . everything is better when it's bigger . . . most of us think 8 cylinders are better than 4 cylinders (except when it's time to fuel up), a 12 oz. rib eye is better than an 8 oz. rib eye and well you know . . . bigger is always better, right?
All kidding aside . . . as others have mentioned, you can always build a small fire in a large firebox if you just need a little bit of heat to take the chill off a cool Fall or Spring Day . . . or even on a cool 4th of July in Maine. Just build a small fire in the firebox with some low BTU wood and voila (or walla if you would . . . refers to another post) you have a small fire in a big fire box that provides a little bit of heat.
Of course, you cannot do the opposite. If it's mid-January and the temps are below the donut and the winds are howling and your smaller stove is crammed to the gills with oak and cranking to beat the band and it's still not warm enough, it isn't really physically possible to build an even bigger fire in that fire box.
One of the best bits of advice I ever got from hearth.com . . . and there have been lots of good advice . . . is to figure out the size of the firebox you need based on square footage . . . and then go one size larger, realizing that there may be especially cold days. I took that advice and I've been very happy with my purchase as it pretty much heated my entire house last winter.
The problem with going with a small firebox, as mentioned, is that burning too hot for too long to try to stay warm (and in some cases failing to stay warm) is not good for your stove (stress), not good for your wood supply (you can go through more wood) or for you (you end up feeding the stove too much, do not have long burn times, worry about your stove's condition, worry about your dwindling wood supply and on top of this are never truly warm).
I like Highbeam's "low throttle" description. This seems pretty appropriate. A woodstove working well in terms of providing good heat and not producing copious amounts of creosote and other pollutants in my opinion should be working, but not working too hard. If you don't get your woodstove up and running hot enough you risk creosote and other pollutants and do not get any meaningful heat. If you over tax your stove by running it full bore too often you risk stressing the stove's components and over-firing.
A similar analogy could be seen with cars. If you're in a race and choose to race with a 4-cylinder car you may find that for some races (heating days) the car works fine . . . but when you face an 8-cylinder car (that mid-winter day in January with the sub zero temps and wind blowing at 20 mph) you will find that your car's engine is redlining (which of course can lead to over heating, stress on the engine, etc.) . . . and on top of this you will probably lose the race. In a similar way, too small a stove may work well on some days, but when you're faced with those inevitable really, cold days in middle of winter, a stove that is too small just will not work for the reasons listed previously.
If, on the other hand, you go with something way, way too big for this race . . . say a V-12 Lamborghini . . . you will find that the car you've bought for the race is wicked expensive and about 95% of the time you do not really need all that power and speed . . . and in fact you could be considered a bit wasteful with both the up-front cost of the car, need for premium fuel, etc. In terms of woodstoves, buying too large a stove is an extra cost considering you do not really use the full potential of the stove . . . and could in fact not be running the stove very efficiently.
Now, if you went with a decent 8-cylinder car . . . you get the best of both worlds. Sure, you may not need 8 cylinders worth of power . . . but it sure is nice to have those extra cylinders there when you need them. In the same way, a stove that is just a bit larger than what you need means you can burn smaller fires (i.e. use less gas using my car analogy) on those shoulder season burns, in normal winter burning you can burn normal sized fires . . . and on those wicked cold days you can use your stove to its full potential by loading it up that well seasoned oak or hickory and maximizing your stove's (or car's) potential.
