What's the legal length of a knife to carry in Canada?

As some of you might know, I've been betrayed by my ex-boyfriend and my so-called mother, when they confessed to me that they had behind my back. I got myself a new identity and I'm planning on starting a new life in the city. However, I'm a little worried about muggers and all that. So my friend who I'm staying with now (he's a ninja in training) recommended me to buy a knife, a big one to fend off attackers with. A single sided blade is considered a tool, and he went into great detail, but he said that if it's too long then it's illegal. How big is too big for a civilian knife?