Real life size is what would be very good. I think anybody would want that to see what his or hers final board is going to look like
Real life size on a monitor is quite the opposite the CAD-systems (either mechanical or electrical cad) are invented for.
Imagine a board with 0201 passive elements (0.6x0.3mm size) and fine pitch tracks (0.07mm track width and clearance). And you would want that in physical dimensions on the monitor? So you are really sharp sighted, I would need a magnifier glass in front of the monitor.
A different issue is if you want to get an additional overview (for instance at the end of the board layout, or to get a feeling for the complete board/system. Than maybe a 1:1-representation between monitor ↔ physical size could be useful for some people.
For this case every individual kicad system has to be calibrated between monitor-size ↔ display-resolution ↔ physical size to calculate the individual zoom-factor for this installation which shows a 1:1-physical representation.
If you really want that, look at the thread Custom scale for a true PCB view
and my answer #8 (step-by-step desription, note: step10 has be proven to not work)
and video at answer #20.
You shoud remember the estimated zoom-factor, so you can use it for every project on the same system.