Captions, like other display copy, are generally styled in contrast to the running text for aesthetic and practical reasons: The appearance of display type offers a change of pace from the uniform appearance of running text and makes it easier for readers to distinguish it from the default content. The font size should be the same as the running text, or smaller.
It’s best to minimize the different forms a caption can take in one publication. When captioning a headshot photograph of a person, for example, be consistent about whether you simply label the photo with the person’s name or whether you provide additional information. Because such photos tend to be small, printing the subject’s name alone is recommended. But a caption for a larger candid photo, as opposed to a posed headshot, for example, should provide some context.
Choose between using incomplete sentences or complete sentences when providing more detail; it’s better form to be consistent with one approach or the other.
When writing captions for images in textbooks, magazines or training materials I always try to include as much information as possible so visual learners will find a way into the information detailed within the body of the content. It’s also useful to include key terms in the caption and bold them in the body copy. Easier for visual learns to scan the pages and find the information when they want or need it.