openbox: change text and "box" size when switching virtual desktops

when I switch between virtual desktops, openbox shows briefly a "box" with icons of virtual desktops and desktop name (number). This is ridiculously small on my large monitor, I can barely read the desktop number: where in the openbox configuration file (rc.xml) can I change the size of this box and the text size of the virtual desktop number? I would be happy if it only showed the desktop number in large font size