Well about what to buy .. it depends on how much space you want and how much space you need ... and if you want to use the external hard drive for a daily or for backing up your files.
If you want to use this external unit all the time and you have a laptop is better if you buy something compact that will not going to occupied too much space and heavy to carry everywhere, but they are normally more expensive. If you want something for your desktop space should not be a problem, but tried to buy something that you can stand the external hard drive so it will not occupied to much space flat.
About size the biggest size out there is one terabyte, that is 100 gigabytes. games will occupied somewhere in between 2-4 gigabytes, for music 1 gigabyte can be around 240 songs.
For movies (an estimate):
MOVIE 1: 1 hour, 13 min - 141 MB (.mov format)
MOVIE 2: 1 hour, 35 min - 552 MB (.mp4 format)
MOVIE 3: 1 hour, 26 min - 417 MB (.mp4 format)
MOVIE 4: 2 hours, 2 min - 532 MB (.mp4 format)
MOVIE 5: 1 hour, 26 min - 169 MB (.mov format)
MOVIE 6: 1 hour, 38 min - 189 MB (.mov format)
MOVIE 7: 1 hour, 57 min - 230 MB (.mov format)
MOVIE 8: 1 hour, 42 min - 196 MB (.mov format)
MOVIE 8: 1 hour, 29 min - 172 MB (.mov format)
MOVIE 9: 1 hour, 26 min - 179 MB (.mov format)
MOVIE 10: 1 hour, 26 min - 1 GB (bought from iTunes)
MOVIE 11: 1 hour, 26 min - 958 MB (bought from iTunes)
MOVIE 12: 1 hour, 17 min - 148 MB (.mov format)
MOVIE 13: 1 hour, 35 min - 533 MB (.mp4 format)
MOVIE 14: 1 hour, 37 min - 195 MB (.mp4 format)
Hope this helps.