I think a 75g tank would do you fine for a long time. These guys do get big, but they aren't really active swimmers like other fish. They spend the majority of their time tucked away in a cave or under some driftwood. But when you toss some food in the tank, it'll get pretty active. And mine will swim around the tank at night. But they are still mostly bottom swimmers, only swimming up into the middle or surface area of the tank infrequently.
I started mine in a 20 long (was very misinformed when I purchased), and he got up to about 10 inches in there. I then transferred him to a 125g tank, where he has been very happy. He's now 16 inches long, and living happily on play sand.
BTW, I have my violet goby in freshwater. I understand that there are several species of violet goby, some require brackish water some don't. But I think they can all live in brackish water either way. Just wanted to make sure you knew that...