Which prefix refers to something that is 'large'?

The prefix that refers to something that is 'large' is "macro-." This prefix comes from the Greek word "makros," which means "large" or "long." In various scientific and medical terminologies, "macro-" is often used to indicate a larger scale or a greater size compared to normal or typical measures. For example, in biology, "macroorganism" describes large living organisms that can be seen with the naked eye, as opposed to microscopic organisms.

The other prefixes do not carry the meaning of large size. "Meso-" typically refers to something that is intermediate or middle in size, "retro-" indicates a backward direction, and "patho-" relates to disease or suffering. Thus, "macro-" is the precise choice to convey the concept of largeness.