Inside the VM, do not use the driver for the physical monitor. Use a "Generic LCD 1920x1080" (or, whatever the Windows equivalent is).
After that, set the resolution to what you want.
Other replies are right...with the VBoxAdditions installed in the guest, one of the features it provides is dynamic screen resizing...as you change the window, it will automatically change Window's screen resolution to match (again, do not install any physical monitor drivers).
Running if full screen (Right-CTRL-F, I think) with the Additions installed should take the display to 1920x1080 (as long as the host, PCLOS, is running that resolution).