Basically, panels of sub VIs can be called in a sub panel of the main VI, no matter where the sub VI is located and as long as the main VI can find and open it. That gives you a handle to the panel in the sub panel on which you can apply methods or control property nodes, such as those which control the panel size and/or those which shift the origin.
I have found a solution using a sub-panel. First, you must enable the developer mode to get the hidden brown palette called subpanel: remote panel:insert remote vi. To create that palette, you need to create a property node of the subpanel and then find the insert remote vi under the method.