Widget API: FontString:GetStringWidth
Returns how wide the string would be, in pixels, without wrapping:
width = MyFontString:GetStringWidth();
Get/SetWidth() and Get/SetHeight() operate very differently on FontString objects than they do in other objects.
Among other things, if no width is explicitly set, GetWidth() will return the exact same result as GetStringWidth().
See the FontString object information for details.