Captive bred doesn't matter... they still get big. A 10 ft male will be average, and obviously the females get bigger.
And even though the plan for babies is in theory, I'm guessing it will stay that way. Mostly because of anaconda breeding habits. You need MANY males and a female to get them to breed....not just one of each. Read up on bredding balls and their wild habits and you'll understand why.
As for the 10 ft vs 20 ft care... they are not that far apart. Aside from more space to move/swim, the diets will be close (and cost roughly the same). The bigger the snake, the more safety you need to plan in (handling, cleaning, enclosure design/strength etc...).
I'd say this thread is more of a pipe dream than anything.