There isn't a way to manually adjust the size of the box. The height of the box will automatically adjust to fit the content. The width of the box will adjust to fit the window.  If you increase the width of your browser the boxes will adjust to match.

If you have short periods, such as "Lunch" or "Recess", you may have better luck setting your planbook so the days go across the top. This is the default setting, but you may have changed it when creating your planbook.

For more information about changing the orientation of your planbook, follow this link:

http://help.planbookedu.com/knowledgebase/articles/106839-how-to-change-the-orientation-of-your-planbook