[smoothshot]

It depends on what version of BlueStacks your running. I found that the version I was running didn't reference what was in the reg file. Check the entries under
[HKEY_LOCAL_MACHINE\SOFTWARE\BlueStacks\Guests\Android\FrameBuffer\0]

If you see
Width
Height
You will need to adjust those entries to match the dwords on WindowWidth and WindowHeight. Or you can just copy the code below in notepad and save as all files to 860x720.reg.. Let me know if this worked for you.

[HKEY_LOCAL_MACHINE\SOFTWARE\BlueStacks\Guests\Android\FrameBuffer\0]
"Width"=dword:0000035C
"Height"=dword:000002D0
"GuestWidth"=dword:0000035C
"GuestHeight"=dword:000002D0
"Depth"=dword:00000010
"FullScreen"=dword:00000000
"WindowState"=dword:00000001
"HideBootProgress"=dword:00000001