Setting widget size relative to parent. Really that hard?

I have been a FF user for a couple of months after starting with zero Flutter experience and limited web design knowledge. This is really basic stuff but I cannot wrap my head around this: is there really no easy way of setting the size of a container relative to its parent?

This issue has been raised multiple times, like here and here, with no or limited response. All with workarounds via "Flex values" (weights). But really, isn't this super basic stuff? Shouldn't there be a way to give the width or height as say "90% of the parent"? What am I missing here?