It's best to buy the biggest you can afford.
I currently have a 200GB external drive which I got last year which I thought i'd never fill. But now, I only have about 35gb left on it.
Mainly its because its got about 80gb of TV recordings (although from my windows laptop, a G3 400MHz isn't powerful for TV). But the rest of my stuff is music, video, photos, downloads and backups.