The name "Small Medium at Large" is a catchy title describing the Reverend Miriam B. Young. Reverend Young is the pastor of "The Spiritualist Church of Glen Meadows", a church which emphasizes "making connections between this world and the other side". The word "medium" has a number of different meanings, one of them being a person who facilitates communication between people and the spiritual world. A person might go to a "spiritual medium", for example, to communicate with someone who has died.
Reverend Young is a spiritual "medium" by trade. She is a woman, and probably rather diminutive, which would explain the adjective "small" before her description. "At large" is a term indicating that something is on a large scale. In the case of Reverend Young, the addition of "at large" in her description would probably indicate that she is offering her facilitating skills to anyone who might need them, the public "at large". The title "The Reverend Miriam B. Young: Small Medium at Large" is actually a newspaper article heading that Cletus has found in the local paper. The heading creatively uses three related words usually referring to sizes - "small, medium, and large" - with different meanings, common, yet with unexpectedly original connotations when used together, to describe the Reverend Young and her expertise (Chapter 7).