I think what most people don't realize is how much lugging you will have to do.
Even though a 15 or 17" laptop may sound cool and very good for spreadsheets, imagine carrying that around all day long to and from school and from class to class. Add to that a power adapter.
I used to have a number of 15.5" laptops in the past but have transitioned to 13.3" size for a number of reasons:
1. Weight/size
2. Battery life (longer with smaller screen, assuming the same battery capacity)
3. Fits better on airplanes (may not be a factor for you)
4. I use a dual 24" monitor setup at work/home, so I am satisfying my need for large screens.
I have to say, I don't miss the screen real estate at all, unless I am working on several documents at the same time and need dual screens...
I don't see any issues with performance for the normal "office" tasks - I have a ton of windows open and it all still works just fine. Right now I have 39 windows open (Outlook, word, excel, Access, IE, Firefox, Fireworks, Google talk, Explorer). No issues.