Helen you are busy today :) If you are talking about the playground or another place - you could say What's the square footage? What's the total area? How big is it overall? Obviously, you can change the unit of measurement to suit - acres, miles, kilometers etc. That is presuming you are asking about area and not length. For something smaller - a square - How big is the square? What size is the square? Do you know the measurements/size of the square? The sentences in your questions are great and very natural Hope this helps Jane :)