DEFINITELY!
In fact, a clever observation of these visual aspects of a text is one thing that can set your response apart. Keep in mind, though, that the task refers to how "language" is being used to persuade and don't go overboard!
Typography is an important element of many forms of persuasive texts. The font style carries with it connotations and connections that the graphic designer and/ or copy editor is well aware of.
Similarly, the text size: a large font is much more imposing than a timid little one and can represent or create, mood, atmosphere or tone.
Just be aware that unless the text is a blog or a forum post then the writer is unlikely to have made decisions regarding font style or size -- it is more likely to have been the person responsible for layout and other graphic elements. So don't say "The writer uses a bold font to..."
Instead: "The choice of the unembellished Arial font adds gravity to the content of the heading..."