I found the answer.
When I set the property 'Font' (group 'Text properties'), Codesys proposes a list of size in point (pt). But, the real value behind is in pixel (px).
Unfortunately, Codesys doesn't show the unity of the size. And we can see a different value of the size thanks to the tooltip (16px == 12pt, http://websemantics.co.uk/resources/font_size_conversion_chart/
The property 'Size' (group 'Font variables') wants a value in px. But the wanted height is expressed in point (pt).
There is a conversion between pixel and point depending of the DPI of the device.
Moreover, the size in pixel corresponds to an abstract square (EM) where each characters of the font is designed. So, I can't be sure that the text has a height equals to my input size.
Thanks to the FreeType library FAQ: https://www.freetype.org/freetype2/docs/ft2faq.html#other-bbox
I set the pixel size to 8×8, but the resulting glyphs are larger (or smaller) than that. Why?
A lot of people have difficulties to understand this topic, because they think of glyphs as fixed-width or fixed-height ‘cells’, like those of fonts used in terminals/consoles. This assumption is not valid with most ‘modern’ font formats, even for bitmapped-based ones like PCF or BDF.
Be aware that the character size that is set either through FT_Set_Char_Size() or FT_Set_Pixel_Sizes() isn't directly related to the dimension of the generated glyph bitmaps!
Rather, the character size is indeed the size of an abstract square, called the EM, used by typographers to design fonts. Scaling two distinct fonts to the same character size, be it expressed in points or pixels, generally results in bitmaps with distinct dimensions!
Note that historically, the EM corresponded to the width of a capital ‘M’ in Latin typefaces. However, later improvements in typography led to designs that greatly deviate from this rule. Today, it is not possible to connect the EM size to a specific font ‘feature’ in a reliable way.
This topic can be closed.