Minimum, for one DME...90gal. with massive amounts of rock. I have over 200lb. in my 90gal. Females will max out at 32" (which mine is) and Males will reach lengths of 36". If you're wanting a DME with other tank mates, I'd say minimum of 125gal, depending on what other fish you want to keep.
Very long life span...some scientists believe 20 - 25 years, so be in it for the long haul. Thru testing, mine is believed to be 11 years old.
The only reason my fish are in my 90gal. is because I just moved and am currently staying with my parents. I do not believe their floors can hold either my 200gal. nor my 210gal. tanks...so, they're sitting dry in the garage.