The maxlength parameter is the maximum length of the text returned by the Marquee object, not counting HTML formatting tags. You will need to experiment with this a bit to get the right width. A good starting point is to use the width of the marquee frame divided by ten.
So for a 400-pixel-wide window, start with 40 and then adjust as necessary. It’s okay to specify a length slightly larger than the frame width, but if you specify a much longer length, it will slow down processing and increase memory usage.