Currently there are three places in the UI which contain a list of font sizes which could be applied to the content, and all of them could be customized separately:
- RadRichTextBoxRibbonUI - could be customized by directly editing the content of the ribbon in the XAML file.

- FontPropertiesDialog - could be customized by replacing the whole dialog (as SDK is available: https://github.com/telerik/xaml-sdk/tree/master/RichTextBox/CustomFontPropertiesDialog ), or a little hacky approach using the FindName method:

((ListBox)((Control)this.radRichTextBox.FontPropertiesDialog).FindName("fontSizeListBox")).ItemsSource = new List<double> { 10, 14, 16, 18, 20 };

- SelectionMiniToolBar - could be customized by changing the template of SelectionMiniToolBar

Provide API which allows customization at a single place. This API could be similar to (or even extension of) the already available Telerik.Windows.Document.Layout.FontManager class.