While it's not clearly in the rule book Ryan Laukat confirmed to Sam that titans do not take up space.
I have to disagree with the "not clearly in the rule book" part.
Additionally, the player must have enough capacity to hold the empire card. Capacity is granted by District cards and Titan cards and a player's capital city (found on his player board).
It clearly says that only empire cards take up capacity slots. Also, logically a card which grants capacity can not take capacity itself.
When one misses this part and the question comes up during the game, it is easily found back in the rulebook as the capacity symbol is printed right under it.
But we all miss these small things while reading rules for the first time, don't we?