Yeah, I think Easterns and Texans have about the same maximum size potential, the Easterns maybe runing a couple of inches larger, but not really significantly, The listed record lengths that I've seen showed the Eastern at about 103 inches and the Texan at about 100 inches as I recall.
Not to split hairs, but you listed the Yellowtail and Texans sub-species names in your question. Corais is the Yellowtail, erebennus is the Texan. The Eastern is Drymarchon corais couperi. Since you said Eastern, but used corais as the ssp name, I assumed you were actually talking about couperi. If you were asking about corais, then there's no doubt, corais wins that contest hands down. Although I've never seen one that large, I've heard of corais reaching lengths approaching 10 feet.
For reference, the subspecies of Drymarchon are:
D.corais corais Yellow Tail Cribo
D.c.unicolor Unicolor Cribo
D.c.rubidus Red Tail Cribo
D.c.melanurus Black Tail Cribo
D.c.couperi Eastern Indigo Snake
D.c.erebennus Texas Indigo Snake
D.c.caudomaculata (new subspecies)
D.c.margaritae Margarita Island Cribo
D.c.orizabensis ? (no common name)
There are some proposals to change the taxonomy for the Drymarchon group, giving some animals species status (such as the Eastern Indigo because of distinct differences in head scalation) and regroup others under D.corais sspdue to various morphological differences. Wolfgang Wuster has a good article regarding the proposed changes that can be viewed at www.indigosnakes.com.