Before getting into how you can adjust the length and scrolling speed, are you planning to use this like a teleprompter?
I'm asking because I did a series of educational videos and we used a free teleprompter website for the scripts. I had 2 cameras mounted on tripods focused on my client so I was free to run the teleprompter controls on a wireless keyboard.
That was ideal because we sometimes had to change the size of the text (or how much was onscreen at any moment), and I also had to adjust the scrolling speed constantly to match my client's energy level and not stress her out or hold her back.
In my experience, manning the teleprompter was a full time job during each shoot and there's really no way to make those kinds of on-the-fly adjustments if you're playing back a produced video.