Container's actual width and height

Hi,

How can I get Container’s actual width and height in px? In designer I can see only default sizes, but I created a CSS-element (Mark Down), so I need to know the actual size of the container.
For now my result is:
In designer:
Pump_Designer
In browser:
Pump_Browser

Project Browser:
PumpStruction
PumpControl is a Template which I paste in another View

I use FLEX containers.
The problem is, DIV elements don’t understand the width and height in % when I use for example border-left property…

Thanx in Advance!