New Harddrive

My dad has a 40GB hard drive and it's constantly full. I've convinced him to get a new one so that he doesn't have to delete things every month. The thing is, what's the difference between a 2.5" and 3.5" hard drive? Is that the radius of the actual disk inside it? What does that actually mean and what one will he most likely need?

Thanks in advance everyone,
Steven