You are not going to get to 1000 square miles, if that's what you're asking. :-) It's all imaginary numbers, but our default measurements are 2 meters per pole on heightmaps, and ten meters per game unit. But you could call it 458 meters per pole, or .001 meters per pole, as distances are all relevant in the virtual.
If you simply want to know how many heightmaps you have placed and how big they are, check the property panel for the heightmap, and check the asset panel to show how many instances you have.