From what I've seen in designer
you can't really select the subwindow, only the subwindow's content widget, so that's probably what your picture shows. So you only can change the subwindow's properties directly from your code, not in designer.
Example:
for subwin in mdiArea.subWindowList():
subwin.setMinimumSize(289, 366)
subwin.setMaximumSize(289, 366)