Earlier this year, I got a 500gb external USB Western Digital drive.
Think it was about $79, at the time. But $89 is a common price range, too.
So, unless it's physically too big for your needs, buy as big a drive as you can afford.
I'd be careful about 750gb and 1.0 TB hard drives, though.
The receivers may or may not be able to handle a drive that large.
Now, having said all that, I used to use an external 60gb USB unit based on a little laptop drive when I first got my FTA receiver.
As long as you keep track of what you are doing, it's big enough (I thought so).
For SD, around 1gb/hr to 2gb/hr, or so.