I haven't kept these fish personaly, but i read post and i here of people keepin them in 55g so a 75g should be a good size. These fish swim for miles in the ocean, so a long tank is recommended and also bigger is better
A 75g is fine for a yellow tang. And that's my experience, not just something I read somewhere from some guy that heard it from somebody else. Bigger is always better, but that doesn't mean a 75g is too small.