Determining how (by adjusting font, font size, image scaling, etc.) the delivered amount of text, illustrations and other matter can be fitted into the allocated page size and extent.
More details can be found under casting off, which is the other side of extent calculation.
This definition is extracted (and expanded on) from the book Getting Published: A Companion for the Humanities and Social Sciences by Gerald Jackson and Marie Lenstrup. It was first referred to in the blog Getting Published in a post on calculating book length.