I had recently owned a new phone (Onplus One) on cyanogenmod 12 (api 5.1.1) and I have exactly the same problem on this one. It has a 5.5" screen.
Screenshot link
well people seems to be able to 100% control it
using layout and QSizePolicy (sizePolicy ) and others settings, but I guess its a trial and error process.
I have never seen option for settings as %. (only in my wet dreams)
or you can override
protected:
virtual void resizeEvent(QResizeEvent *e);