Like Chris asked, where is it going, and what are you burning? Logs?
The space is the most important aspect, in terms of the base level of insulation and movement of air. You don't want to be underpowered, burners have maximum output ratings that are not the real day to day output. You want it to accept fairly large logs, sawing everything up is a faff and it is expensive to buy smaller cut wood. If you have access to free wood you will want to use it, and a larger box makes sense.
Saying that, if it is for a small shed or something, a cute wee burner can be just the thing.
Always nice to have a hotplate too. Stainless designs are a great idea, but often they are thin walled.