Yes, size 9 font is too small for a resume. Size 9 font is hard to read on a computer, and because you’re likely sending your application over the internet, the hiring manager will have to strain their eyes just to read your resume.
You should instead use a font size that’s at least 10.5 points to make sure your resume is immediately readable. If you go between 10.5 and 12 font, your resume should be clear enough, regardless of your chosen font.
However, the font you choose does impact your resume’s readability, and each font is slightly different in how it looks at the same font size. For instance, the following fonts are particularly hard to read at smaller font sizes:
- Gill Sans
- Italic fonts