I found that I could add fontsizeselect to the Toolbar string, but I'm having trouble defining the sizes somewhere. In the JS settings, there's an "initi" section that allows this. But I'm not sure of the C# equivalent.
If you want to allow editors to choose from a specific list of font sizes then, as you've spotted, you can add "fontsizeselect" to the toolbar to display the font size dropdown. To control the options in that dropdown, you can call the AddSetting method of your TinyMceSettings like this:
config.Default() .Toolbar("bold italic underline fontsizeselect") .AddSetting("fontsize_formats", "1pt 2pt 3pt 9pt 100pt");
To maintain consistency between pages though, I tend to go for allowing specific styles from the style dropdown instead of allowing the editors to change font sizes directly.
I think what you are looking for is here in this documentation https://docs.developers.optimizely.com/content-cloud/v12.0.0-content-cloud/docs/custom-style-formats
I see there are paragraph options, but I don't see an option to set a custom font size. Any ideas?