Where does Drupal save the trim length for the body field?
In Drupal 7, field settings are saved into {field_config_instance} and {field_config} database table.

Trim length for a body field is saved in the data column in the {field_config_instance} table.  The data column is stored in BLOB format, so you are not able to see the value of this. 

You can use the field_info_instance() function to extract information about a particular field in a particular entity bundle/type:

$body = field_info_instance('node', 'body', 'page');  
print_r($body);