I keep Boas and Anacondas in the same environment, they do fine. I even keep an old Ball Python, who nobody wants, in with the small male Anacondas and Boas. Balls can be problem shedders, but in that humidity, when he sheds, his skin comes off like it had a zipper, every time. I would always measure humidity inside the cage, as different types of heat can affect humidity, locally. Circulation is also important. I use small 3-4in., 110v computer fans mounted 12" above floor & outside, on timers. This reduces humidity some. Their mouth, upper airway, and lungs can be damaged by too little. I use LOW voltage ( 12 to 15 volt ), heat pads directly under the water container, which I keep well above air temp, so there's humidity to spare. Do NOT do this with 110volt ( or higher voltage ) heat pads unless you're SURE they are absolutely waterPROOF. OK? Then, still don't do it. I've got a big female who pushes her 50 pound water container to both ends of a 7 foot cage, sometimes it sloshes. You don't want to win a Darwin award, posthumously.
If they are happy, a large cage is not necessary. 6x2x3 sounds fine to me. If you provide for it, they like to climb and perch. It's impressive to guests, to see a big Yellow snake, perched on a 3" diameter hardwood branch. In a 3 ft tall cage, you can install a mezzanine, also.
The lighting intensity isn't important, but the period regulates them.
If I were using that electric radiator as a central heat source, I'd have two of them, both operating at the same time, connected to different circuits, if possible. One cold night with a full belly can be fatal. (now, I'll open my 17th bar of soap of the day). I have areas in my cages under 75 degrees. I often find the snakes there, for long periods of time. I've measured their surface body temp at 75 degrees often, with no respiratory problems resulting. Radio Shack has a good IR non-contact thermometer for surface readings and spot body temperature. In fact, right now, it's $20 off. Thirty bucks is cheap, for that indispensable tool.
I've seen much variance in adult size of Yellow Anacondas. I have one big male over 8 ft, but 'only' 25 to 30 pounds. My largest female is only 10 ft or so, but she is stout, twice as heavy as the male. I also have a young, rescued male, who was abused, and is very long over 7 ft, but with a tiny head. I have another adult male only 51/2 feet, under 10 pounds. They vary a lot. A few years ago, Conan O'Brien had a giant female Yellow Anaconda on his show. She looked to be over 12 feet and thick, and was very tame, but very active under those hot stage lights.
I was lucky to be in on a couple of shipments from New England Reptile Dist., several years ago. They are the only people I have good, first hand experience with, in Yellow Anacondas. I picked out the calmest individuals, but they were all healthy and calm, still dragging their yolk sacks. NERD's website has excellent information on Anacondas. They refused to ship the snakes to us until the weather was right. I'd buy from them again.
As for me, I won't count my babies until they are squirming around. This forum will let you contact me directly, rather than talking buying/selling here.
As for info. resources, you take your chances. I started keeping Anacondas with prior Boa constrictor experience, and with good info. from NERD. Also, learned a lot of what not to do by observing other keepers, locally. Too many people want Anacondas because of the dreaded testosterone poisoning syndrome. Fastest car, girlfriend with biggest...eyes, Pit Bull/Rottweiler, most notorious snake, all to make up for small schmeckle. It's hard on the snakes. Most folks in these forums are not that type, I've found, they're teaching each other. The snakes, and the hobby, benefit from these forums. I don't have resources to rescue any more, so when someone tells me they have an Anaconda ( locally ), I take a walk.
Pic shows why that IR thermometer is handy. I'm on the foggy side of the Cascades, they chill quickly outside. I'm thinking of installing heat on their outside exercise tree ( 18th bar of soap ).
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To Buddhist hot dog vendor. "Make me one with everything."
Just thinking about the 6 foot Green Anaconda I was asked to tame a few years ago, and laughing at myself. That snake had not been handled as a baby because the owners were afraid of its tiny teeth. That snake wasn't mean, just didn't want to give in to handling. It would accept it for a few minutes, crawl all over me, then would bite. It liked to go between my fingers and bite there. OUCH. A six footer has strong jaws. He didn't strike, or hold on, just a calm firm bite. After a couple of weeks, and a lot of blood-stained clothes, I gave up. That's why I was suggesting you get yours tame while she's only three feet. They grow quickly.