Fix RichTextBox Margin Issue

I'm seeking an experienced C# developer to help fix a margin issue in a Microsoft Word add-in I created using Visual Studio.

Currently, there is an unwanted margin on the right and bottom of a RichTextBox (System.Windows.Controls.RichTextBox) hosted within an ElementHost (System.Windows.Forms.Integration.ElementHost).

Requirements:
- Completely remove the right and bottom margins
- Experience with C#, WPF, and Windows Forms integration
- Familiarity with Microsoft Word add-ins development

I've tried some solutions that haven't worked, so I need an expert to resolve this issue effectively.