Edit the file qa-include/db/maxima.php and change the value of the variable QA_DB_MAX_CONTENT_LENGTH from 12000 to a bigger value, e.g 20000
'QA_DB_MAX_CONTENT_LENGTH' => 20000,
This file has maximum values for many variables. You can change them as per your requirements.
'QA_DB_MAX_EMAIL_LENGTH' => 80,
'QA_DB_MAX_HANDLE_LENGTH' => 20,
'QA_DB_MAX_TITLE_LENGTH' => 800,
'QA_DB_MAX_CONTENT_LENGTH' => 18000,
'QA_DB_MAX_FORMAT_LENGTH' => 20,
'QA_DB_MAX_TAGS_LENGTH' => 800,
'QA_DB_MAX_NAME_LENGTH' => 40,
'QA_DB_MAX_WORD_LENGTH' => 80,
'QA_DB_MAX_CAT_PAGE_TITLE_LENGTH' => 80,
'QA_DB_MAX_CAT_PAGE_TAGS_LENGTH' => 200,
'QA_DB_MAX_CAT_CONTENT_LENGTH' => 800,
'QA_DB_MAX_WIDGET_TAGS_LENGTH' => 800,
'QA_DB_MAX_WIDGET_TITLE_LENGTH' => 80,
'QA_DB_MAX_OPTION_TITLE_LENGTH' => 40,
'QA_DB_MAX_PROFILE_TITLE_LENGTH' => 40,
'QA_DB_MAX_PROFILE_CONTENT_LENGTH' => 8000,
'QA_DB_MAX_CACHE_AGE' => 86400,
'QA_DB_MAX_BLOB_FILE_NAME_LENGTH' => 255,
'QA_DB_MAX_META_TITLE_LENGTH' => 40,
'QA_DB_MAX_META_CONTENT_LENGTH' => 8000,