First off, from what I understand it is impossible what you want to do - the VST plugin tells the host (i.e. OpenMPT) how big its window should be, not the other way around - the host cannot simply resize plugin GUIs.
Apart from that, the OpenMPT source code is available under the BSD license
, which means you can do almost anything you want with it. However, if you really want to provide your own fork, you are kindly asked to rename it to something other than "OpenMPT".