My laptop uses the same screen resolution (1366X768). LiveCD of the Phoenix edition, also starts in a 1024X768 and the same resolution is kept after install. But the graphical environment shows with no problem. And I didn't have to be root to change the screen resolution to 1366X768. Just asked for a restart.
So it's the same situation on a different laptop with a different PCLOS Edition. Is it a problem of the KDE or a problem of the laptop's screen?
That particular laptop refuses to display the 1024x768 resolution during live boot up to Desktop.
Another latop I tried displayed OK and the res was easy change.