If you want to see the shock front move on something like a second timescale, I'm pretty sure you need to be close enough to be in real trouble.
Typical expansion velocity is on the order of 10 000 km/s radially, so you get 20 kkm increase in diameter per second. For safety then, 20 kkm should correspond to a barely perceptual angular change, which wikipedia tells me is around 1 arc minute.
So if you are close enough to just see the shock front move, you have about an hour until it gets to you, and if you want to avoid being overtaken you need to be able to move at ~ 10 kkm/s.
Interesting question. My off-the-top instinct would be to say any explosion that looks bigger than the sun is probably delivering more radiation than the sun and that would be bad news. But that’s without doing any math.
The size of the visible object is determined by it's real size and distance... That's to say, no it will never look big.
But it can look very bright, so it illuminates the sky. It would certainly be interesting to have an object so shiny that it creates a glowing halo around it, but I'm not sure it would be safe to look at that halo.
How spectacular of a view could you get from an earth like planet ... and still not have any ill effects?
Could it be a huge spectacular thing reaching across the entire sky, or would by that stage you be in real trouble?