I'm just wondering, if the size of the Earth has been understood since ancient Greek times, it must have been known that there was a large planet that had yet to be discovered. If it's known that the earth is a large planet, then it would also be understood that the Mediterranean and northern Africa were just a small part of that world. What were the ancient Greeks' thoughts when it came to what the rest of the world was like, or did they even ask the question?
They certainly asked this question. In Plato's Phaedo, Socrates compares all of the civilisations around the mediterranean to frogs around a pond, and says it is very likely that in distant places there are many other people: "I believe that the earth is very large and that we who dwell between the pillars of Hercules and the river Phasis live in a small part of it about the sea, like ants or frogs about a pond, and that many other people live in many other such regions." (Phaedo, 109a-109c).
You can read my more detailed answer here.