At that scale the minimum saveable size on my machine appears to be 820x459, which is slightly larger than the ~813x438 you're trying to set (which is the actual minimum size of the window).
It's possible the minimum saveable size varies depending on other aspects.
You could use the "Perform a Window Function" action in a command that runs when your profile loads to force the size you're looking for (provided it's larger than the actual minimum for the window).