To change screen resolution in BlueStacks Android emulator you need to edit two registry items:

1. Run regedit.exe

2. Set new resolution (in decimal):

HKEY_LOCAL_MACHINE\SOFTWARE\BlueStacks\Guests\Android\FrameBuffer\0\Height

and

HKEY_LOCAL_MACHINE\SOFTWARE\BlueStacks\Guests\Android\FrameBuffer\0\Width

Kill all BlueStacks processes.

Restart BlueStacks