And don't forget the myriad uses after a natural disaster. As a wrecking tool, it is unmatched.
But what size do you need? Growing up, my Dad had a crowbar that was about three feet long (that's a meter for my Metric friends) and it felt like it weighed a ton (a tonne)! Maybe I was only nine years old, or maybe it did weigh a lot. Either way, it was very unwieldy. I'd recommend about an 18 inch (half meter) crowbar as a good size. Big enough to provide leverage, small enough not to wear your arm out swinging at the zombies massing at your door. It's big enough to do the required work/bashing, but small enough to carry around easily.
My crowbar is similar to the one I'm showing you below. Happy Bashing!