Yahoo changed it like one year ago or so and now when I start writing an email the font size is extremely tiny and I have to magnify the website in browser settings each time and it drives me crazy.
In the top right corner of yahoo mail hover over the gear, and select "settings" from the menu that appears. On the menu that loads from clicking "settings" you should select "Writing email" in the left column. Then you will see "default rich text format" and the second drop-down allows you to select the size. (Tiny, Small, Medium, Large, Extra Large, and Huge)