What size screen do you recommend and why? For MacBook Pro?

After being frustrated with my PC laptop, I am switching to a Mac. I currently have a 15.4" screen, but was thinking about a 13", more for portability than anything. I am a masters degree student. My work is in accountancy. I won't graduate until May 2011, so I don't think I will have outrageous spread sheets. Let me know what you think.