openbox rc.xml application size

This

<application class="Obconf" name="obconf" type="normal">
  <size>
    <width>45%</width>
  </size>
</application>

or longer

<application class="Obconf" name="obconf" type="normal">
  <size>
    <width>45%</width>
  </size>
  <position force="no">
    <x>center</x>
    <y>center</y>
    <monitor>1</monitor>
  </position>
</application>

https://forums.bunsenlabs.org/viewtopic.php?pid=72844#p72844