Use larger fonts in WordPress Visual and Text Editors
Especially when you are tired you may have a hard time writing a post, with the WordPress editor font size being so small. Unfortunately WordPress won’t let you change that.
The best way to enjoy a larger font size in the editor, is to use a plugin. There is Visual Editor Font Size, simple and working. In the editor page (at the bottom) you will find a “Visual Editor Font Size” option, where two buttons let you increase or decrease the size of the editor’s font, even preview the changes right there, in case the main editor area is not visible.
Of course your actual post will not be affected, it will keep following your theme’s style. This plugin works only on the visual side of the editor, it won’t change the font at the “simple text” pane.
If you need a larger font in both panes, the visual as well as the text pane, try another plugin called WP Editor Tweaks. This tweaker won’t let you choose font size, but it selects by default an efficient large size you will enjoy. According to the author in a future version user options to customize the font will be added. Right now you just activate and play.