While the MLA handbook recommends using “an easily readable typeface” and a font size “between 11 and 13,” it also clarifies to follow a professor’s or instructor’s guidelines if they differ. The handbook advises using double-spacing and the same font and size throughout the paper.
Check with your instructor on their preferences, and in the absence of any such preference, use a decent and readable font, like Times New Roman, with font size 12, which is a good balance between readability and aesthetics. The most important thing is to use the same font and size consistently throughout your paper.