1x and 2x windows are useful for the developers, given we normally have a debugger window or 5 open. Full-screen ones aren't, but can be easily accessed via your window manager's normal widgets.

The "prescale" option has absolutely nothing to do with the window size and should normally be set to 1 unless you like unnecessarily making your CPU slower; hopefully QMC2 (the app) doesn't have the same misunderstanding about it that QMC2 (the person) does :-)