Mathjax is a fantastic plugin for WordPress that renders your latex code in a wonderful way. However as with the other Latex renders the font-size can be a bit small. The CSS field on the settings page is not working for some reason, but there is a simple way to remedy that. In your theme CSS file just add
.MathJax_Display {
font-size: 1.5em;
}
where the numerical value is of course set to what looks good on your site.