Length of Supporting Statement to Faculty Position application
I would not refer to what you are writing as a statement. I would say it is a response. Responses should be as long as they need to be, but not longer.

If they have given you a person specification with 15 points, and ask you address every point, then, yes, your response is going to be long. There should be no need to guess on this. They have asked you to be thorough so you should be thorough.

While it is common for companies to use person specifications to ensure they find a proper match for a position it is rare for me to see someone use it in the manner you describe. However, it does not sound crazy, just like they want to really be sure that they are getting all the information they need, and they clearly want to be thorough. In the end, the goal of the person specification is to make sure you eliminate, as early as possible, those people who are very likely to be a poor match for the position/organization. If you can respond appropriately to each point, then you are much more likely to be called for an interview. So don't leave anything out that answers their questions.