Best Size of MacBook Pro for University?

So I'm heading off to Uni in the fall and have no idea which MacBook to get. I don't like the 13" white MacBook as everyone of them I've seen has been covered in dirt and nasty looking. I also don't want the 17" Pro because it will be too heavy to be lugging around campus. So it's pretty much between the 13" and the 15". Do I need the extra 2", is 13" too much of a strain on the eyes/too hard to type on? Is the 15" too big for a uni desk? Is a 500GB HDD worth it over a 320GB or 250GB? Also do I need any specific programs on it (Microsoft Office?). I'm in the BBA program so I don't think I'll need Photoshop or Final Cut of anything. Please help me. Thanks guys! :D