org-html-mathjax-options

org-html-mathjax-options is a variable defined in `ox-html.el'.
Its value is

((path "http://orgmode.org/mathjax/MathJax.js")
 (scale "100")
 (align "center")
 (indent "2em")
 (mathml nil))

Documentation:
Options for MathJax setup.

path The path where to find MathJax
scale Scaling for the HTML-CSS backend, usually between 100 and 133
align How to align display math: left, center, or right
indent If align is not center, how far from the left/right side?
mathml Should a MathML player be used if available?
This is faster and reduces bandwidth use, but currently
sometimes has lower spacing quality. Therefore, the default is
nil. When browsers get better, this switch can be flipped.

You can also customize this for each buffer, using something like

#+MATHJAX: scale:"133" align:"right" mathml:t path:"/MathJax/"

You can customize this variable.