How big does my Time Machine HD need to be in my MBP is 250GB?

depends how long back you want to keep backups, and how much you move create and delete files.

If you're buying a new external hard drive you'd probably just go with at least 500GB since they are pretty cheap now. That should be plenty of space.

I've was given 1.5x as a guideline before I got my TM drive and it works out to be quite accurate. 250GB isn't huge so maybe go for a 500GB drive. 320 would not be big enough.

Make it big. I originally used a 320GB as a Time Machine drive for my 250GB internal drive. It filled up pretty quick.

Now I just use SuperDuper! to keep a clone of my drive. If I ever have a problem, I just put the clone in and keep going.