Well bobby you do sound rather young? a male yellow anaconda will get over 10 feet. females will get over 14 feet. a cage for them is measured more in feet then gallions. yellows in general not saying all are but most are nippy and picky eatters. by far anacondas are the hardest snake to keep. if u do not devote 100% to them u will have some major problems.
Please don't take my post as an attack on you, it is not. However your post seems more like a post to discourage him rather than give him honest advice.
Your comments on length are not entirely accurate. Those are projected maximums, and while it is indeed possible for an individual to reach or exceed them, saying one WILL is not accurate. In my experience its hard to find a yellow much over 10 feet long period. They are out there, but are not very numerous.
Also in my experience yellows as CB babies are as variable in temperment as any other boid. I have seen them come out as calm as can be and I have seen some real chainsaws. As with most baby boids though, interaction can make them very calm in a short period of time. They are intelligent snakes and gaining one's trust is a matter of showing it you will not harm it, then it will decide if it wants to be friends or not for lack of a better way to put it.
They are not the hardest snakes to keep, no by a long shot. Overall in snakes, yellow anaconda's in my experience are on the easy side of the spectrum. To someone who doesn't have a good deal of experience, yes, yellow anacondas can be a challenge. Start off with a nice CB baby though and your issues will be minimized. With proper care and dedication that you should really give to any snake, a yellow anaconda can grow to be a most impressive snake.
Again, I am not attacking you, but all too often in these KS forums I see posts that try and discourage rather than offer practical advice and facts.
Would I suggest a yellow as a first snake? No. Would I suggest one to a person with some experiece and who is looking to move up so to speak? Yes. They are a good species for that.