It will be about half the size of the moon and clearly visible to the naked eye, and it could last for months.
Betelgeuse is 'only' 650 lightyears away, so when we see it the nova happened 650 years ago, in theory.
They say it won't affect us because of the distance, but the Solarsystem does have a diameter of almost 4 lightyears.
That means Betelgeuse is not that far away at all, actually it is relatively close.