first go to playback setting in default window size select custom 700X400 save, and then try again
Edit:
or first sendkey 1 to exit full screen
Local $hPotPlayer = WinWait("[CLASS:PotPlayer]")
WinActivate($hPotPlayer)
Send("1")
WinMove($hPotPlayer,"",0,0,682,729)
or better
Local $hPotPlayer = WinWait("[CLASS:PotPlayer]")
WinActivate($hPotPlayer)
Send("9")
WinMove($hPotPlayer, "", 0, 0)
Sleep(100)
Send("8")
to hold aspect ratio