Options are discussed here in reply to cocoaaa ,who posed your question. The option that telamonian points out has moved in version 3.2 to Settings → Theme → Increase Content Font Size. Using that now, it didn’t seem to do much very fast, but I think it had changed the setting that buggyprogrammer describes. It was much more straightforward to demonstrate by adjusting to extremes using buggyprogrammer’s method, rather than advancing or decreasing slowly.
Related to that, I came across a way to temporarily adjust sections in the rendered display of the markdown cell that I’ll note here as well because it may be helpful to emphasize only certain parts.
I base this example on what is shown here, I’m using the session launched from here still:
# THIS IS A TEST
<font size="12">your_text_here</font>
Compare rendering with the following in another markdown cell:
# THIS IS A TEST
<font size="6">your_text_here</font>
And then another to compare to a third markdown cell:
# THIS IS A TEST
<font size="3">your_text_here</font>