A resume should be font size 10 to 12. Most fonts between size 10 and 12 make the content of your resume clear enough for an employer to read.
However, some fonts look small at font size 10 (like Garamond), and big at size 12 (like Merriweather). As you make your resume, try different fonts at different font sizes to see what works best. Before sending out your resume to a company, print it out. If it’s easy to read and looks professional, then you’ve chosen the right font and font size.