The biggest tank you can get may suffice, but think about how much water there is per fish in a lake where they are naturally. It's a lot of water per fish. But as said before, if you can do a 300 gal or more tank, it will work for a while, with few tank mates.
As for the legality, it's per state, but it my research, I've found that it is legal to keep a fish that was caught in season, so I'd try to stick to that.
Right now I've got a long-nose gar in a tank, he's 10-12"s now, but they get to 50+, so that's something to consider. We're going to build either a large tank or an indoor pond for him. A problem we've quickly discovered with domesticating a wild fish, is that they eat a lot. In the case of the gar, this is partly due to their preference for warm water (our's @ 85' F), this of course increases their metabolism. Still, this little creature (slightly more girth than a large sharpie), has eaten over 2 dozen feeder fish in a week (some almost as big as he is around).
If you want to keep a wild fish, I'd suggest something of a smaller nature, like a perch or a crappie. If that works out, just build yourself a massive tank and model your local waters.