I will keep all the formula's and equations out, and keep it simple.
Well, according to conventional physics, the Halo would have to be a near perfect ring in order to maintain it's artificial gravity, which would keep all the objects, including humanoids, on it's inner surface as well as keeping the 'atmosphere' in in place. This would be centrifugal force. In order for this to work, the ring would have to be a near perfect ring you see, as it would have to rotate in order to maintain this. Any thing other than a near perfect ring, and the gravity would not be constant, and you would encounter 'pockets' of weaker and stronger gravity in spot, and the ring would eventually tear itself apart. Keep in mind this is by physics to our knowledge. That is the thing with Sci Fi, is you can operate outside of conventional physics. so... The Forerunners, could have some other method to achieve this, like an artificial gravity generator.
By our physics however, mass wouldn't be an issue in this case, as all you would have to do to increase the gravity, would be to rotate the ring faster.
As far as how it would look if you were standing on it, I think they had it fairly close. It's just all about perspective and where you are in relation to the view, considering there are mountains and such.
But I agree with what so many have already said, I don't think they really put much science behind it, they just made it look good.
I could be wrong, but that is just my thoughts on the subject.