The part of Lake Ponchatrain crossed by the Crescent from Slidell is the narrowest part of the lake and is only a 5 mile crossing. It's similar to the biew you get from the nearby I 10 crossing, which I did once, and it was pretty impressive. But I would think if you really want to get an idea of how big the lake is, stay in New Orleans an extra day, rent a car and drive over the Causeway, which is a 20+ mile crossing. Maybe some tour operators run bus tours across, I don't know that,but they should.
If you drive back through Ponchatula and I 55, you'll see that most of I55 is a bridge crossing a swamp. In fact, it seems that the only way you can get to New Orleans is by crossing a bridge. It's kind of like the Manhattan of the South.