size of drive needed for time machine backups

I have an iMac with a 500 Gb disc. I have owned the machine for a few months now and am considering getting a portable external drive for backups. So far I have only used about 50 Gb of the disc after transferring most of my stuff from my old PC and cannot see myself using much more than that for a long time. Most of the advice I have seen says you should get a backup drive at least twice the size of the disc to be backed up. My question is do I really need a 1 Tb backup drive bearing in mind the fairly small amount of data to be backed up? Would 500 Gb portable drive not be enough? Thanks.

The 500 Gb disk will be enough, however, you also need to consider how long you wish to keep the backups. Some folks are paranoid about keeping backups forever which of course will fill even a very large drive after a while. I personally see no reason to keep my backups longer than 6 months. Backed up data becomes less useful over time.