(I ask this out of pure curiosity—it will be a long, long time before I would even consider taking on the challenges of keeping any semi/aquatic snake, let alone an anaconda. XD)
What is the average size and/or general size range of a male yellow anaconda? I’m seeing tons of conflicting information when I try to search it. 
Has anyone here kept them before?
I have not personally kept them, known people that did and usually they were not very large around the 6’ maybe 7’ at most and slender compared to a female which can get in the 10’ or a little over range. All this is just the typical, it also depends a lot on your feeding schedule, just like with other boas and pythons.
Good to know about the relative weight… I love anacondas as a whole, and the wild-type yellows in particular… but have drawn a line at acquiring any animals I can’t realistically (and safely) handle and care for alone. If I ever end up in a different living arrangement, and there are other members of the household who have any interest in snakes… that’s subject to change, lol. XD
In any case, this highlighted point in particular interests me… I honestly never knew about this phenomenon until recently. How does that work? Are snakes just biologically wired to grow more or less, relative to the abundance of food? If so, that’s a pretty useful adaptation… Reptiles never cease to amaze me. XD
Basically you could say that! Genetics also come into play. Your yellow anaconda for instance will never get as big as a green anaconda, or a mainland retic. But definitely environmental factors such as abundance of prey does play a role in size. I would never want anyone to try and stunt growth of a reptile by starving it, but you can feed a appropriately sized prey item to a baby snake every 4-5 days, weekly, every 10 days or bi-weekly and be feeding your snake correctly! I would say especially there first few years when there growth is much faster then later in life can be impacted by food amounts. No one has a 20’ retic that didn’t feed it alot!
Wow, that’s incredible… I never knew snakes could do that! I figured that size was 100% up to genetics, barring actual stunted growth! How do snakes perceive “hunger” then? I know there are cases where they will refuse to eat for a stretch of time, for various reasons… but does that mean that “hunger” to them is more instinctual prey drive, as opposed to what mammals experience? Or just that they can go longer without actually suffering, but would still take food if it were convenient (provided they weren’t still processing their previous meal)?
Ah, so that’s why I see a severely obese snake from time to time? The owner was trying to achieve maximum potential size, but way overshot? I always wondered how you could overfeed a snake, when there’s not really a practical way to free-feed them. XD
A lot of snakes including corn, ball pythons, boas, the list keeps going… are overfed a lot in captivity. It is hard for people to understand that they do not have the same metabolic rate as other animals and don’t have to eat frequently. You are right, most snakes will eat if the prey is abundant and can be somewhat opportunistic predators. On the other hand some will only eat well during a bird migration or nesting season on islands for example, others just eat a few large meals a year! I would never suggest these extremes in captivity but you can skip or reduce size of prey if your snake is getting too obese, obesity is taxing on a snake’s health and can ultimately lead to premature death. Now the reason all those feeding scenarios I gave for baby snakes is they do have a tremendous growth rate in there first few years and unless you are power feeding(making them eat whenever they will take food) you will be feeding them normally. Then as a snake ages, depending on species of course you can give them a larger meal less frequently or smaller meals frequently, you just have to pay attention to their body composition and health. Also most snakes do go off feed and is usually nothing to worry about, most won’t even lose a significant amount of weight. They do this for a variety of reasons, breeding, stress, unknown reasons. Just keep your husbandry on point and offer appropriate amount of prey and they will usually take care of the rest! Have you ever had larger type snakes before? I only ask because any anaconda is I wouldn’t say extremely difficult to provide proper husbandry, but definitely not a beginner larger snake in my opinion-from a husbandry standpoint. From a temperament standpoint I have heard good and bad, but that’s true of most any snakes. This needs to be taken more seriously though with larger snakes, just so they are able to be worked with properly, without neglecting their needs.
Oh, thanks for all the info! I definitely won’t be rushing into anacondas—I know they have more particular needs than the more beginner-friendly species, and I wouldn’t want to get in over my head. 
Everything I’ve heard/seen seems to emphasize that they’re harder to read than most snakes, in terms of body language, and that’s what results in more incidents of striking? As for temperament itself, I’ve definitely seen a range of opinions/accounts, as well. I’m guessing the truly hardest part in keeping them would be adequately meeting their aquatic needs, right? I’ve seen really beautiful enclosures with deep “swimming pools” for the snakes, and then an equal amount of naturalistic, dry land. I can only imagine what goes into cleaning that water… and keeping it clean. 
Mostly, I wanted to know how typical (or not) it was for a male to stay in the 4-6’ range, so that I could decide whether or not to rule an anaconda out entirely (as a snake I could manage single-handed). I will definitely be starting out with something known to be a little more forgiving, and save that for when I have more hands-on experience, though. XD Yellow anacondas are my favorite species, but that just means I wouldn’t want to keep one unless I knew I could really give them the best life possible, and provide a great space that meets all of their needs. 
Totally agree with you, some great species to consider that may help you “prepare” for a yellow are bci, bcc boas, rainbow boas, and on the python tip- short tailed pythons, carpet pythons. 