The editor widget provides two functions for setting size, by integer points or integer 100th points, [see](https://www.scintilla.org/ScintillaDoc.html#SCI_STYLESETSIZE) and Geany uses the integer one [here](https://github.com/geany/geany/blob/9c91c287803ae05ce0f2aacd605564301e0ef4aa...).
If somebody provides a PR for a function that uses the 100th point API it should be possible.