I started using "GianT" when I started DJing 1995. Then I used it on IRC and for playing games (quake) where giant was a well known nick after a while.
Well nowadays I sometimes wish I would have chosen another nick, because when people see someone with the nick "giant", they probably think "oh that guy thinks that he is the greatest/best". So I thought pretty often about changing my nick, but I would need new email etc. etc. etc. so I've still kept it.
Also giant fits nice to me because I'm pretty large (2m), so when people meet me in real-life they always are like "oh now I understand why you call yourself giant!".

That's my story...