The value is hardcoded unfortunately :-
Code:
/opt/zimbra/jetty-6.1.5/webapps/zimbra/js/zimbraMail/mail/view/ZmMailMsgView.js
ZmMailMsgView.OBJ_SIZE_TEXT = 50; // max. size of text emails that will automatically highlight objects
It is calculated based on the message size / 1024.