Ravnica is an ecumenite: a city from all over the world.
The largest map of Guide to the guild master in Ravnica shows only the 10th arrondissement. Its diameter is about 10 miles, which makes it as big as Amsterdam. Assuming each district is the size of the tenth, the whole world would be as large as Luxembourg.
To illustrate how small this world is: if you had a sphere the size of Luxembourg, the horizon distance would be about 250 meters. And if you were riding a decent horse, you would go around the world in 5 hours.
The only logical conclusion I can draw is that there must be parts of the world outside the 10 districts. But what is it, desert? Oceans? Other neighborhoods?
What would I find if I ventured out of the tenth district?