Big text field GUI.

So i'm making a RPG in Orthographic projection, and can't really figure out how to make a big string or how to make a GUI.Box with alot of text in it.

I need the following: When someone says something to my Player i want a GUI.box (Later with textures and such) to appear with a couple of lines of text. Is there a way to do this easily? or do i just have to make a really long string in one line or (alternatively) make ALOT of strings? If my game is going to have a decent amount of verbal stuff i can't imagine how many strings i need then. The way i would do it was something like:

 if(Talk == 5)
 {
   GUI.Label(Rect([position doesn't really matter]),"Hello, I am John the Smith...");
 }

I thought something like a .txt doc it pulls the string out of. But i'm not familiar with that kind of stuff at all and the possibilities with that. Thank you in advance.

-CDK