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