Hi Ian,
Are you talking about the thumbnails in the Pages pane?
In that case, you can right-click on the Pages pane to bring up the context menu.
"Enlarge Page Thumbnails" and "Reduce Page Thumbnails" are items on that menu.
You can also click on the little gears icon under the name of the Pane. That bring up the same menu.
Regards,
Charlene