In real life, an A4 paper is smaller 24=16 times compared to an A0, or 6.25% of A0 size.
When I start a new document in Scribus with the size A0 and set the zoom to 10%, in theory it should be larger than an A0.
In reality, it is like an A7. Can you explain what's happened?
If you need "actual" size on the screen, you would have to play with the zoom setting. Start with 100% zoom. If it isn't accurate, adjust the zoom setting until it matches an actual ruler.
when you scale it to 6.25%, the size in the screen should equivalent to a real A4 size.
scaling to 50% will shrink a line to half its length. but for an area it will be a quarter.