var style = document.Styles["Normal"];
style.Font.Size = 5.0;
tm = new TextMeasurement(document.Styles["Normal"].Font.Clone());

string someText = "I like to eat at McGruderalpostermus.";
cell.AddParagraph(AdjustIfTooWideToFitIn(cell, someText));

private static string AdjustIfTooWideToFitIn(Cell cell, string text)
{
    Column column = cell.Column;
    Unit availableWidth = column.Width - column.Table.Borders.Width - cell.Borders.Width;

    var tooWideWords = text.Split(" ".ToCharArray()).Distinct().Where(s => TooWide(s, availableWidth));

    var adjusted = new StringBuilder(text);
    foreach (string word in tooWideWords)
    {
        var replacementWord = MakeFit(word, availableWidth);
        adjusted.Replace(word, replacementWord);
    }

    return adjusted.ToString();
}

private static bool TooWide(string word, Unit width)
{
    float f = tm.MeasureString(word, UnitType.Point).Width;
    return f > width.Point;
}

/// <summary>
/// Makes the supplied word fit into the available width
/// </summary>
/// <returns>modified version of the word with inserted Returns at appropriate points</returns>
private static string MakeFit(string word, Unit width)
{
    var adjustedWord = new StringBuilder();
    var current = string.Empty;
    foreach (char c in word)
    {
        if (TooWide(current + c, width))
        {
            adjustedWord.Append(current);
            adjustedWord.Append(Chars.CR);
            current = c.ToString();
        }
        else
        {
            current += c;
        }
    }
    adjustedWord.Append(current);

    return adjustedWord.ToString();
}

string someText = "I like to eat at Mc-Gru-de-ral-pos-ter-mus.";