static void Main(string[] args)
{
    List<string> lines = WrapText("Add some text", 300, "Calibri", 11);

    foreach (var item in lines)
    {
        Console.WriteLine(item);
    }

    Console.ReadLine();
}

static List<string> WrapText(string text, double pixels, string fontFamily, 
    float emSize)
{
    string[] originalLines = text.Split(new string[] { " " }, 
        StringSplitOptions.None);

    List<string> wrappedLines = new List<string>();

    StringBuilder actualLine = new StringBuilder();
    double actualWidth = 0;

    foreach (var item in originalLines)
    {
        FormattedText formatted = new FormattedText(item, 
            CultureInfo.CurrentCulture, 
            System.Windows.FlowDirection.LeftToRight,
            new Typeface(fontFamily), emSize, Brushes.Black);

        actualLine.Append(item + " ");
        actualWidth += formatted.Width;

        if (actualWidth > pixels)
        {
            wrappedLines.Add(actualLine.ToString());
            actualLine.Clear();
            actualWidth = 0;
        }
    }

    if(actualLine.Length > 0)
        wrappedLines.Add(actualLine.ToString());

    return wrappedLines;
}

添加WindowsBase和PresentationCore库。