This might be completely useless but it's worth a try: try overriding the OnResize method of the form (or the Height/Width or Size properties if possible) and throw an exception there. Then look at the stack trace to see what caused the resize.

maybe something like this
Code:
Protected Overrides Sub OnResize(ByVal e As EventArgs)
   MyBase.OnResize(e)

   Throw New Exception()
End Sub
If I'm not horribly mistaken execution should halt here (on the exception) and you can view the stacktrace in the exception dialog.