Hi, I won't make claim to being an expert or breeder of mandarins though I have bred them. I have had them for a number of years and they are not all that hard to keep. Gathering advice from those who kept them before me and for the internet has made them pretty easy to keep. I'll try to answer your questions as best I can.
"What is their average and maximum adult lengths?"
I have a breeding pair that are about as big as a good sized corn snake, say 4' each. I have seen pics of individuals that appear to reach over 5'.
"What kind of temperment do they have?"
Generally calm but can be tempermental thowing hissy fits slapping thier tail or abruply jerking thier body. It can be comical as its not as much a warning as it appears to be a temper tantrum. 
The breeding pair have can be voratious feeders and I have to becareful to avoid being bitten around feeding time. They will calm down to be held, though the female has once calmly turned and chomped down on my wrist. Not because she was mean or was out of sorts, but because she simply was hungary. Mandarins can be king snake stupid at times.
"What kind of husbandry techniques are called for?"
Cooler temps are a must, around the low 70's down to the high 60s. They are kept as low as I can in my reptile room. No extra heat is provided beyond the ambiant room temp. They are somewhat secretive so I give mine aspen shavings to bury in and a hide.(the breeder pair do take turns prowling though) In the vision cage they have a large plastic coffee can that works well for my other snakes but prefer to coil in or under a simple page of newspaper laid on the bedding. Because these snakes are kept cooler they do not eat as large a meal and as often as american rats thier size. Feeding them heavy may comprimise thier health.
I know people who do and thier snakes grow fast power feeding but its always a risk with any snake and more so with mandarins.
"Do they go through a hibernation period?"
If your breeding them yes. I'll separate mine to sterolite tubs and place them on the floor dropping thier temps in the low 60's for about two months in the winter feeding very small mice 3 weeks apart, if they want it at all.
"if you breed a pair, what kind of clutch sizes are to be expected?"
I got a clutch of 7 eggs this year which I was told is unusually high as 4-5 are more common. Unfortunately I ended up with one baby as they were laid when I was away and were not noticed by the person I had caring for my collection. Only two hatched and the first of them died that day. The lone survivor is strong and beautiful though. Note that incubation temps are lower also being in the mid 70's as opposed to 80-82 for most other rat snakes.
They are in some ways esier to keep than some other snakes like pythons or other tropicals that suffer if they get a chill. I think if you get a pair you'll be happy.
If you buy online and ship a pair, wait for cooler temps as if subjected to heat they won't make it.
Hope this answers your questions.
Good luck with them, Mike.
