Why was it impossible for spain, or indeed any other european nation, to realize the extent of the territory spain controlled after may 1493?

I think that "May 1493" refers to the papal bull "Inter caetera" , which divided the world into two parts, giving the western part of the world (today almost all Americas except a small part of Brazil) to Spain.

At the time, the Americas were not yet mapped by the Europeans, so they did not know whether it was a small island or a huge continent-  the correct answer is that the whole Americas have not yet been visited by the Europeans.

No clue what is the answer

Idon't think any of em are right, you need to add the number of what paragraph you got your citation from