How about this US Bureau of Land management report from 1982 which cites the size of the Ixtoc I spill as 3-5 million barrels (linked as a reference on the wiki entry):
http://www.gomr.mms.gov/PI/PDFImages/ESPIS/3/3930.pdf
Or a NOAA incident report which provides duration and flow rate estimates for the spill which translate to the same total spill size estimates as other sources (3+ million barrels):
http://www.incidentnews.gov/incident/6250