Two clicks via Menu > File > Properties or ALTF+R or two keys at the same time with two fingers CTRLD is one of the quickest shorter key combinations.
Whilst M can be floating values to show where your mouse is on the page which is usually more useful than the authors original or intended boundary values.
Every page can of course be a different size but most documents in Europe tend to be the same ISO size ratio that’s the beauty of golden ratio pages, there is no need to describe sizes just use the step size that’s big enough for that scale.
Dimensions only have meaning once it’s committed to paper so most of us would be able to guess if its best output at A5 or A4 without any screen clutter. Why would it matter since in a virtual world zoomed dimensions have no relevance until printing when you change it to what’s needed at that time.