I think the largest ones we've seen were around 18" arm-to-arm, 6" mantle. Norman lists "body size to at least 12cm, arms to at least 35cm" for bimaculodes, the large-egged version that would be the tank-raised ones, and "body to 20cm, arms to 80cm" for bimaculatus which are small-egged so they're generally not tank-bred, but hard to tell apart so sometimes may show up as wild-caught.
Not much lately. Marinebio_guy moved to Hawaii, so AFAIK he's not raising them (maybe he'll start sending cyanea, though?), no news from Zyan's dad recently, and Octopets is sadly out of business. It also turns out that it's technically illegal to collect bimacs from the wild for resale in the pet trade in California (although it doesn't seem to be enforced much, so they sometimes show up anyway.) We've seen hummelincki sold as "bimac" because they have two spots, even though they come from a different ocean, recently on occasion, so it's not clear that what suppliers say is what they send you.
Oh, you mean do they hide all the time? Bimacs are one of the more outgoing and interactive species, once they're not babies and used to people, they're frequently out a lot, and will come out when they see you hoping they'll be fed. Or pull the inside cleaning magnet so the outside falls and gets your attention. But as with most pets, their personalities are variable, too.