I dont know anything about the elephants at Six Flags Safari but maybe this site will help a little. Its a nice database for elephants all over the world.
Perhaps the most striking finding is that enclosure size is secondary to social structure in ensuring optimal welfare. The importance of (readily modifiable) management and enrichment regimes is also stressed.
As the authors write: "caring for zoo animals is still more of an art rather than evidence-based". This research is exactly what zoos should be doing.