Scientists are predicting It'll be the size of the moon from our point of view and visible just about everywhere in the world-though Is the radiation close enough to actually harm us?
Betelgeuse is about 200 pc away, and its axis of rotation doesn't point anywhere near us so we shouldn't get a huge gamma burst. The UV burst is expected to be weaker than the amount of UV we get from the Sun routinely, so that shouldn't hurt anything either. About a hundred thousand years after the supernova, we'll get hit with the proton flux, and that could have some consequences. But that's a LONG time off even if the star flares in the night sky tonight.
Also, "the size of the Moon" is an error. It will be brighter than the Moon, but it will still be a pinpoint; even if the gas cloud is driven outward at a significant fraction of the speed of light, it would take at least decades before the resulting nebula reached the apparent diameter of the Moon, by which time Betelgeuse itself will be a neutron star and invisible to the naked eye.