"Now suppose we come up with an acceptable definition of "big enough" and "long enough", which will be a compromise definition. How do we enforce this definition? Punishment. "

You enforce it with fees against cable TV service providers, or whoever is responsible for publishing the advertisement.

"So, suppose we come up with a suitable punishment for these law breakers. Who will enforce this punishment? What if they do not pay the fine? Do we throw them in our already crowded prisons or build new prisons? "

The idea is not to punish people, but to bring more fairness to the consumer in terms of the information they are given and not to be mislead. If a cable company can get a way with displaying a novel worth of information for one second (in order to comply with disclosure requirements) and in the process duping a customer into signing up, I think there should be some rules against that.

As for enforcement, the FCC can easily enforce something like this.