Video files add up in a hurry. That all depends on the length of the file you will be using. I use the following to figure it out.
[Video Bitrate (# bits/sec) + Audio Bitrate (# bits/sec)] X # Seconds = Total Files Size (# bits)
Just put in your info and do the math and you will have a good idea of how much space you need. A 500 GB HD is a good idea to start. You can get buy fine with 250 but the extra space is very nice. I'm constantly fighting with space on my HD. I can't remember the last time it wasn't full.