The option 'page width' specifies a fixed page-width in px or a page width as a percentage of the article frame width.
Pages with fixed width usually make texts easier to read because the individual lines in a text are guaranteed not to be too wide.
Pages with width set in percentages adjust automatically according to the dimensions of the article frame.
Center page in frame
When this option is active, the page will appear in the center of the article frame.1
The outer margins determine the distance between the borders of the exported article frame, and the borders of the article-page. 1
Inner margins (padding)
The inner margins determine the distance between the text inside the exported article-page, and the page borders. 2
1. The left- and right-outer margins are ignored when the page is centered.
2. Inner margins are ignored when the page is set not to be visible (see Page).