There are two possibilities to set the width of a label using max-width and 
max-screen-percent-width in <pos> forcing word-wrapping. None of those two work 
(neither in 2.4.4.7-r1 nor in 2.6.0.4-r1) like described in http://www.jirka.
org/gdm-documentation/x1259.html

I searched through the sources of greeter and found item->data.text.
real_max_width as the place where max width is read for wrapping. But the only 
place it is set to an new value is when the label is located in a 
vertical-oriented box. This is a good workaround for the problem (if you keep in 
mind that xpadding and ypadding of the box are ignored!?! :) Internally also 
max_width and max_percent_width (preset is 90%) are set but I haven't found a 
place where real_max_width is calculated from these values (I did not a deep 
search).

Are there plans to (re)activate the functions? Or will there be something else 
in future releases? Another problem is, that in user messages and translations 
there are hard-coded newlines ("\n") - looks sometimes strange. Also the height 
of a label is calculated as for one line of text causing the wrong height after 
wrapped text is displayed (but you can manually add an empty label to reserve 
the space).

All not real problems but quite confusing when making a theme.