<pd4ml:page.header>: Header Height

PD4ML offers 3 different ways to define headers/footers, which is sometimes confusing.

First please take a look to the description:
html-pdf-faq-f1/pdf-page-headers-footers-definition-options-t41.html

It seems you mixed up (inline header, supported by PD4ML renderer directly) with (JSP custom tag, mapped to PD4ML API call).

implementation is close to any HTML block tag, so you may just use height or style attribute to define the header area height.