FT_ESTIMATE_UNITS tries to determine the units of a geometrical object by
 looking at its size and by relating this to the approximate size of the
 human head according to the following table:
   from  0.050 to   0.500 -> meter
   from  0.500 to   5.000 -> decimeter
   from  5.000 to  50.000 -> centimeter
   from 50.000 to 500.000 -> millimeter

 Use as
   unit = ft_estimate_units(size)

 This function will return one of the following strings
   'm'
   'dm'
   'cm'
   'mm'

 See also FT_CONVERT_UNITS