TFM allows you to modify the design size of a font in two different
ways. The first option is to override the original design size at
load-time, by calling load-font with the :design-size key.
For example, (load-font "cmr10.tfm" :design-size 12) will load
the 10pt Computer Modern roman font at a design size of 12pt. The other
option is to change the design size of a font later on, simply by using
the design-size writer, for example like this: (setf
(design-size font) 12).
Note that you can always revert the font to the original design size, since that value is remembered (see Font Information).
In any case, the design size must in principle be expressed in TeX point units, and must be greater or equal to 1. One exception to this rule is if you are not working with TeX points. In such a situation, you could probably hack your own custom design size in, to express the font’s various metrics in another unit of measurement.