<<Or at which scale% do i have picture size 1920x1080 ?>>
If you right-click the clip and select "Set to Framesize", that will set the clip to 50%. Of course any scaling of that to 100% is done with no quality loss. (don't use the similar named 'Scale' option)
There is an user preference under "Media" where you can set it to "Set to frame size" and it will set UHD footage to 50% when dropping into a HD timeline.