Go to your admin screen and there click on appearance and Custom Content (HTML/JS/CSS)

0_1516725655010_ee6485ac-8c0b-45df-97d3-b2310dc9ce19-image.png

and there use this:

body { font-size: 14px; }

Change the number 14 to 15, 16 17 or 18 to make the font size bigger 🙂