The following configuration let openbox open all windows maximized per default:
edit ~/.config/openbox/rc.xml and insert in the <applications>-tags:
<applications>
<application class="*">
<maximized>yes</maximized>
</application>
</applications>
reconfigure running openbox (Desktop --> rightclick --> reconfigure) or restart openbox.
Now all applications open in maximized windows.
further informations: http://openbox.org/wiki/Help:Applications
If you want to maximize only a special application window, use this:
start the application, check the application window-parameters with obxprop | grep "^_OB_APP" and insert in the <applications>-tag the corresponding code (e.g. evince PDF-Viewer):
<applications>
<application class="Evince">
<maximized>yes</maximized>
</application>
</applications>
reconfigure running openbox (Desktop --> rightclick --> reconfigure) or restart openbox.
Now only the selected application opens in a maximized window.
further informations: http://openbox.org/wiki/Help:Applications