Jennifer,
I agree with Joseph that generally speaking it's easier to have the student use custom software of ever just magnificent options built in to all web browser, but I also have an answer to the specifics of your question. In Quizzes with multiple choice questions, the default answer blank is just a blank. But, if you click the pencil icon to the right of the blank you'll get the Rich Content Editor including the text formatting options that it has available. If you are using the old (current) Rich Content Editor it looks like the size and style options are not available so you'd need to click the html editor link and manually change it for larger font size. If you enabled the New Rich Content Editor you can easily change the size of the font.
Having said all of that; you would need to do this with every answer choice in every question. A better option is to leave the font sizes alone on individual items within Canvas and rely on the user's systems to make everything larger to meet their needs.
Rick
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