Check out the AUIWIN.BP in [908,28]. It illustrates several uses of XCALL AUI, AUI_WINDOW, including one that calculates the size of a text string (in pixels and millicolumn/millirow units).
(Admittedly, this is hard to find in the documentation, if at all.)
If you don't have the program, you can download it from
http://www.microsabio.net/dist/soslib/