Firebox volume
Firebox volume, in cubic feet, is the size of the chamber that holds the wood — the best single proxy for a stove's size class. Under 1.5 cu ft is a small/cabin stove, around 2 cu ft heats a mid-size home, and 3+ cu ft is the large-home, overnight-burn class.
Volume drives two things: peak heat output (more wood burning at once) and burn duration (more fuel loaded per reload). It also sets the log length the stove accepts — a 3 cu ft firebox typically takes 18–20 inch splits, a small stove maybe 14 inches.
Bigger is not better if the space is small: a large firebox run at a trickle to avoid overheating the room burns dirty. Match the firebox to the space and climate, then use emissions and efficiency to choose within the class.