Well, you could ask that, Dr. A., but I think you're complicating things. Why not just ask "How much land do you own?". It's much more simple and causes no confusion whatsoever.
As for what a piece of land over 500 acres is called, I'm not sure there's an answer. It could be a tract ("parcel" implies a smaller piece of land). You could also say a "parcel" of land. None of these, however, is the definition of "over 500 acres".
In Canada, we often measure by "hectares" which is 2.471 acres. After that, we have quarter-sections (160 acres), half-sections and sections.