You can change the font size of the ColumnChooser as shown below:
radGridView1.ColumnChooser.Font = new Font("Segoe", 15, FontStyle.Bold);
More information about working with ColumnChooser is available here: https://docs.telerik.com/devtools/winforms/controls/gridview/columns/working-with-columnchooser#accessing-the-column-chooser-instance
I hope this helps. Should you have any other questions, I will be glad to help.