I think you would be well-served with a 5' boxblade. I would not go too much larger. A 5' will cover your wheel tracks.
How durable a boxblade to get depends on what you expect from it.
For example, I use mine to grade my driveway, and a neighbor's. I also use it for light landscaping work and, with the rippers all the way down, use it to rough "till" my garden. I would consider my use to be fairly light duty. Accordingly, I bought a locally produced boxblade that is approximately equivelant to the light duty models sold at Tractor Supply. These models can be had for about $350.
Heavier-duty models can be quite a bit more expensive, but the concensus here seems to be that they are definitely worth it if you need that kind of capability.