Try these, and you may adjust the font-size that fits you the best.
.tile.calendar.agenda .main-content { font-size: 1.75em; }
.tile.news-feed .main-content { font-size: 1.75em; }
Please note these CSS snippets provided are not officially supported and you may have to adjust them if there are DOM changes with future releases
-EDIT-
Updated the css slightly to apply the font size change to the whole agenda content, as well as including the font-size change for news-feed tile.