it's simple bro.
1. make a dup of the reg file provided n rename it "1280x720"
2. right click n choose edit
3. replace the entire text with this:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\BlueStacks\Guests\Android\FrameBuffer\0]
"WindowWidth"=dword:00000500
"WindowHeight"=dword:000002D0
"GuestWidth"=dword:00000500
"GuestHeight"=dword:000002D0
"Depth"=dword:00000010
"FullScreen"=dword:00000000
"WindowState"=dword:00000001
"HideBootProgress"=dword:00000001

4. OR just edit these 2 values:

"WindowWidth"=dword:00000500
"GuestWidth"=dword:00000500

5. DONE!!!