You can change the size of the thumbnail pane by dragging the border between the thumbnail and page panes.
Sizing thumbnailsYou can make thumbnails smaller to get a better view of long documents or to easily select multiple pages, or make them larger to get a detailed view of each page. On the View menu, click Thumbnails, and then click the size you want.
You can easily reorder the pages in a multi-page document by dragging thumbnails to a new position.