Check the datasource settings in CF Administrator. Make sure CLOB or "Enable long text retrieval (CLOB)" is checked.
How are you "calling from jQuery"? Is it a form "post"? (Do not use "get".)
Which version of ColdFusion are you using? (I ask because some newer security features have been added in newer versions of ColdFusion.)
When posting to a varchar(max) field, be sure to use CF_SQL_LONGVARCHAR.
Do you have any ColdFusion features running that would sanitize the submitted content? Depending upon the type of content being submitted, you may need to disable scriptprotect or some of it may be removed or altered.
@Jamo - Good points!
@isccsi, have you checked the AJAX data that jQuery is sending to ColdFusion using your browser's developer tools or a sniffer like Fiddler or Charles? Make sure that your jQuery code isn't truncating the data before it gets sent to ColdFusion. Once you rule that out, maybe log the length of the value arriving in the MyField argument to see if it is arriving intact to the function.