Between fractions and subscripts using MathJax characters can get really small to the point of not being legible under the current default zoom conditions. From here it was recommended that I see if others on the EE SE are interested in changing the default size/zoom of equations here.
Here's an example text of a dual subscript in a fraction that makes it really hard to read. Even the "t"s in it are difficult to discern. Your votes will determine if I just need new glasses.
$$ \frac{V_{t_p}}{V_{t_n}} $$
Here's the quote from the other Meta stack site stating a way to change the default size:
Or, if sufficiently many EE users think that the font size in formulas should be increased, that can be done site-wide, by changing MathJax configuration. E.g., the configuring script could include MathJax.Hub.Config({ "HTML-CSS": {scale: 120} });. This is something you can bring up for discussion on meta.EE.
EDIT: No one had come up with a good answer/solution to the original question on the other SE so this question was originally a feature request. I've changed it to be a support topic as no new feature is needed. W5VO answered this question perfectly in his comment.
\large
or\Large
to increase the size of a line. This is useful if you are "going down the rabbit hole" with subscripts. See here: forkosh.com/latex/ltx-178.html \$\endgroup\$