Change Font.Size in a RichTextBox .SelectionFont with multiple FontFamily

How can I change the Font.Size in a RichTextBox .SelectionFont (wich has two or three different FontFamily), without affecting the FontFamilyand the FontStyle

This one works fine, if i have only one Font.Family

richTextBox1.SelectionFont = new Font(richTextBox1.SelectionFont.FontFamily, 12);

But, if I have more, it throwing nullexception.

Thx