This is just like most other computer software. When you set the size of a font to be 12 points in your word processor, you aren’t directly setting the size of any particular character. That is all down to the font geometry itself (i.e. ‘W’ is wider than ‘I’). The point size of a font is a very indirect measure, related to the vertical spacing between lines. It just acts as an overall scaling factor, in the same way when in PsychoPy we effectively specify something similar using a variety of units. When we say the text is 1 cm high, that doesn’t mean that a
G and an
o will somehow become the same physical size. Their height relationship is governed by the chacteristics of the particular font itself.
So in essence, in PsychoPy, as in most other software, we can’t directly control the size of individual letters or glyphs. We can only specify an overall scaling factor for all glyphs that the font contains. And even then, our scaling factor (as with all other software) refers to a vertical scale, which is proportionately applied to width.
EDIT: but actually after re-reading your question, I’m not entirely sure what it is that you are asking. What precisely is the p[roblem that you need to solve?