Hi,
There is a known bug/problem in KDE Plasma
The KDE developers don't have time to solve this bug, so users will have to wait or fix the problem themselves. You can go on the bug page and report that you also noticed the bug and hope the kde developers will fix it. I sent an email to the person in charge of the bug, but the kde developers don't have the time for this which is strange because it has a high impact on usability and adoption/testing of kde on virtual machines (if you don't know how to fix the problem).
I don't blame the developers, maybe they have other things to worry about and there is always the manual fix and maybe as long as the bug is present, more people will learn about the fix.
A person already said in this page that the culprit is Kscreen.
The but can be fixed manually:
" KDE Plasma5 can't change resolution in VM..." . Here is the bug report link:
https://bugs.kde.org/show_bug.cgi?id=407058
The bug report also has the fix for the problem inside the comments.
To fix the problem you have to disable KScreen2.
To enable changing the resolution in PCLinuxOS live iso inside Virtualbox, here's what I did:
I disabled KScreen2 from Start Menu > Background Services > Startup Services and then I increased the display resolution from 800x600 and the change was persistent.
The resolution didn't immediately reset to 800x600 if KScreen2 is disabled. Even if KScreen2 is enabled but its Status is "Not Running", the resolution cannot be changed, so make sure to disable KScreen2.
I expect the fix will work with VMWare, too.