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.