When your uprez'ing to DCI 2K from Standard 2K files, oh how many times am i going to refering to these documents in the future :-)Here's the complete DCI spec, section 3 defines image size seems they are using full aperture width.

http://www.dcimovies.com/DCI_Digital_Cinema_System_Spec_v1.pdf
http://www.dcimovies.com/errata/ERRATA.pdf