If you are using Godot 3 and a ttf font file, the proper flow of creating a custom font could be:
- Create a DynamicFontData file.
- Create a DynamicFont file using the DynamicFontData file.
- Use the DynamicFont in any Control nodes.
In this process, we can change the font size in DynamicFont properties -> Settings. But if you want to use the same font with a different size anywhere else, you have to duplicate this DynamicFont file, adjust its font size and use that. Don't modify the properties of existing font file.