How can I find out how big an opponent's realm is?

In Crusader Kings II, Catholic rulers can ask the Pope for a licence to ki-- er, invade, and take the top-level title (Duke, King, Emperor, God-King, etc.). You can only do this to invade people bigger than you, so your best bet is to build up your realm until you're just slightly smaller than the other guy and then request the invasion. How do you tell when you're just a little smaller than the other realm? There's a nice number in the upper-right corner of the screen that tells you what your total realm size is, but how do you find it for other rulers?