I'd say that the size depends on your ground and what you plan to do with the box blade. We need some more information here. Is the ground rocky? Clay, sand, or loam? Are you going to scarify or spread gravel? How uneven is your land? Rutted? Grassy? Any shrubs? What plans?
BTW, In snow, a box works better pushing backwards than forwards. You'll use it that way more than you think in dirt too, so make sure that the cutting edge will reverse so that it cuts agressively when pushing in reverse....some advertisements are misleading and use "reversible" to mean the cutting edge that is bolted on has a second cutting edge available by unbolting and flipping the edge when the first edge dulls. That's something most of us probably never will do, but it's an inexpensive option on most cutting edges - On the other hand, when the back of the box has a blade that cuts forwards as well as in reverse is a wonderful thing to have.....but more rarely found. Often that requires a flip-down cutting edge mounted backwards
On tires, I've run turfs, lugs, and R4s. I prefer the R4s....when new they are almost as good as lugs, but R4s lose their newness fairly quickly and become more like a lugged turf tire.
Enjoy!
rScotty