I don't know if it's a limitation of Geany itself or some other library, so please accept my apologies if the issue is not related to Geany.
Many apps allow setting different font sizes, and Geany is not an exception. However, I miss the option to increase/decrease font sizes by 0.5 (which is possible with some other apps). For example, I use 9pt size for editor, and it's a bit too small, but 10pt is too large. Of course, it also depends on font family (some fonts don't need adjustment by 0.5pt to make them look "right"), but this feature would be useful with many font families.
I can manually enter font size 9.5 into Geany settings, and this value is accepted and saved, but actual font size is not changed. No matter if I enter 9.1 or 9.9, actual size is still 9.