Typeface typeface = new Typeface(txtTest.FontFamily, txtTest.FontStyle, txtTest.FontWeight, txtTest.FontStretch);
double len = MeasureString("テスト", txtTest.FontSize, typeface).Width;

/// <summary>
/// 表示文字幅を取得する
/// </summary>
/// <param name="text"></param>
/// <param name="fontSize"></param>
/// <param name="typeFace"></param>
/// <returns>表示文字サイズ</returns>
private Size MeasureString(string text, double fontSize, Typeface typeface)
{
    var ft = new FormattedText(text, System.Globalization.CultureInfo.CurrentCulture,
                               FlowDirection.LeftToRight,
                               typeface, fontSize, Brushes.White);

    return new Size(ft.Width, ft.Height);
}