Since no answer was satisfying for me, here is my solution:
private void RichTxt_TextChanged(object sender, TextChangedEventArgs e)
{
string text = new TextRange(richTxt.Document.ContentStart, richTxt.Document.ContentEnd).Text;
FormattedText ft = new FormattedText(text, System.Globalization.CultureInfo.CurrentCulture, FlowDirection.LeftToRight, new Typeface(richTxt.FontFamily, richTxt.FontStyle, richTxt.FontWeight, richTxt.FontStretch), richTxt.FontSize, Brushes.Black);
richTxt.Document.PageWidth = ft.Width + 12;
richTxt.HorizontalScrollBarVisibility = (richTxt.Width < ft.Width + 12) ? ScrollBarVisibility.Visible : ScrollBarVisibility.Hidden;
}
Question is about performance depending on text length and how often it is refreshed.