In regards to the firebox size, I would look for a stove that you can load N/S (i.e. the cut ends of the splits facing the door) - or allows both E/W and N/S.
N/S loading is easier, and it's easier to stuff the box as full as possible (longest burns).
If you can do both, you also have the option to slow the burn a little by going E/W.
My point of view on this is that you'll be reloading the stove every day. Not having to reach in to put a first split E/W in the back of the stove (over hot coals), but just being able to stick them in N/S is a major convenience.
Not to say that E/W loading can't be done nicely (see the exceedingly well-stuffed boxes of
@Caw elsewhere by using rectangular splits) - all I am saying that I have done both, E/W and N/S. I would never buy a stove that can't do N/S (if I have a choice - sometimes, especially with inserts, choice in dimensions is limited).