Well, it's either set in the database in the race_info table, or the one crystal space makes up for the model. I'm guessing it's a rectangular box, as there are three fields in the database.
The crystal space bounding boxes are mostly not very accurate, on a test server check the height of the label above a male ynnwn, then compare it to the height above a dwarf (if I remember right). The label is a fixed distance above the bounding box, see src/client/entitylabels.cpp:303.
And if I recall correctly, the height of the ylianm model is about 1.8 units or so, so take the units to be meters and you should be ok.