The common size for male sliders is around 8 inches. Females can reach between 8-10 inches, and the RECORD size for a slider is just under 11.5 inches. Do not come in here throwing around absolute false information.
As for everyone else, back off the guy. He clearly has it housed poorly now, but he's taking steps to fix the problem.
That being said, a 75g is what's commonly recommended for a male RES, at 90 for a female. These are adequate for the turtle in question, depending on gender. Don't forget that without proper filtration, a tank this size just makes your job harder. So start thinking about a good size canister filter.
Josh H
You must be talking length because that is a tiny size for an adult red ear slider.....