Page width and height (paper format) is given as a conversion parameter. By default it is A4.
HTML is rendered using htmlWidth parameter (an equivalent of web browser frame width).
By a conversion, htmlWidth is mapped to given paper width (minus horizontal margins).
A height of page (from HTML perspective) can be computed from the above info.
See http://pd4ml.com/cookbook/pdf_page_formatting.htm for more details.
But I would think you are trying to go a wrong way. Probably it would make more sense to enclose logical parts of document to
sections and let PD4ML break the content to pages.