exchars             - How many characters to return, actual text returned might be slightly longer.
                      The value must be no less than 1
exsentences         - How many sentences to return
                      The value must be between 1 and 10
exlimit             - How many extracts to return
                      No more than 20 (20 for bots) allowed
                      Default: 1
exintro             - Return only content before the first section
explaintext         - Return extracts as plaintext instead of limited HTML
exsectionformat     - How to format sections in plaintext mode:
                       plain - No formatting
                       wiki - Wikitext-style formatting == like this ==
                       raw - This module's internal representation (section titles prefixed with <ASCII 1><ASCII 2><section level><ASCII 2><ASCII 1>
                      One value: plain, wiki, raw
                      Default: wiki
excontinue          - When more results are available, use this to continue
exvariant           - Convert content into this language variant`

Get a 175-character extract:
  api.php?action=query&prop=extracts&exchars=175&titles=Therion