So, for school, (photoshop, illustration, cinema 4d, some final cut pro X) should I get the 15", or the 13"?
15 for power but 13 for mobility i guess?
I also might want to use external monitor, will the graphicscard make that better in the 15?
Not to contradict VanillaCracker, but those are all pretty "graphicky" applications you'll be using. In your case, getting the 15" would be worth it for screen real estate alone. The fact that the 15" will presumably come with a more powerful GPU than the 13" is icing on the cake.
I also don't find the 15" particularly big or heavy at all, but I'm coming from a 17" MBP so I suppose it's all relative.