80GB can be way too much for some people and laughable for others. A friend of mine has 2 external 500GB hard drives FULL of files and music, so 80GB to him is nothing. To other people however they will never even fathom of filling up 80GB worth of hard drive. I have a 500GB external hard drive because I have easily exhausted the 80GB hard drive of my laptop. They're quite easy to use. Instead of saving a file or music or whatever to your main drive, you just direct the save at the external drive. It really is that sample. With the new Time Capsule you can do it through WiFi making it even easier.
My MacBook Pro has a 250 GB Hard Drive, and I've used 97 gigs of it. Mac Mac Mini only has a 60 gig Hard Drive, and needless to say, that's used up. I store all of the Mini's music, videos, and photos and an external drive.
It depends on what you want to use it for when it comes to size.
you may already know this, but if you're referring to the MB Air, you should be warned it only has one USB port. So, if you have an External HDD hooked up all the time, that means you have ZERO USB ports to use.
80 GB could be too much for a college student, or it could be too little. It depends on what your using it for. My cousin has a MacBook Pro and he is into design and things like that, that use PhotoShop, etc. His MacBook is full and he has an external harddrive with 500 GB of memory that is full as well...so all in all, it depends.