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
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
Or Fortune magazine: http://money.cnn.com/galleries/2010/fortune/1005/gallery.exp...
Or a 1981 case study by the Royal Swedish Academy of Sciences, by way of JSTOR: http://www.jstor.org/pss/4312725
Good enough for you?