[He does it for goth cred points. Also me crying because now I have to make a bunch of stuff up because Oda just threw the fact that it's cursed at us with no explanation. Law thinks for a moment.]

We found it while exploring an island shortly after entering the Grand Line. I picked it up because Shachi and Penguin were afraid of it, and when it didn't curse me I figured that was a sign.
[The wording makes it sound like he did it for their sake when really they had been begging him not to touch it and he did it to bully them. Anyway.]

The size isn't really an issue for me since I don't really use it like a regular swordsman... The fact that it intimidates people is useful too.