MathJax can enlarge any mathematical expression that it has typeset. To make this happen, you must first enable a Zoom Trigger using the MathJax contextual menu. You can make the trigger be a click, or a double-click, or even just hovering the mouse over an equation. You can also specify that you must be holding down a key like the ALT or OPTION key in order to trigger the math zoom.
Once you have selected the trigger, you simply have to use it over any typeset mathematics, and you will get an enlarged view. Clicking anywhere in the browser window will remove the enlarged display.
Try clicking on this mathematics (the trigger has been set to “Click” for this page):
$$
\left( \sum_{k=1}^n a_k b_k \right)^2 \leq
\left( \sum_{k=1}^n a_k^2 \right)
\left( \sum_{k=1}^n b_k^2 \right)
$$
Once you have set a zoom trigger, it will be remembered for the site where you set it (unfortunately, you will need to set again it for each site you use). The setting from this help page will not be remembered, however, unless you explicitly set it yourself using the contextual menu.
In addition to the trigger, you can also set the zoom factor for the enlarged view. By default, it is set to 200%, but you can make that larger or smaller, to suit your preferences. If the mathematics is too big for the window, the zoom display will include scroll bars so that you can see the hidden portions.
Enlarging All Math
In addition to zooming individual equations, you can change the size of all the mathematics on the page at once, so if the math is too small initially, you can enlarge it in relation to the surrounding text. This is also accomplished through the contextual menu: in the Settings submenu, select Scale All Math... and type the scaling factor (a percentage) into the dialog box that appears. The mathematics will be re-displayed using the larger (or smaller) size.
As with the other menu settings, the math scaling factor will be remembered and used on the other pages on the site where you have set it.

