Well, here are some ballpark answers. I don't have a burm, but I read and forum lurk a lot.
>>what is like an avg size in captivity for adult burms
Fully grown, between 13-20 ft. Longer has been recorded.
>>what size cage do you keep a full grown in
8 ft long is common, don't know other dimensions. Bigger if possible.
>>what are they eating like rabits
Youngsters get rats, big ones get rabbits.
>>how do you hold an adult burm
With at least one more person assisting you. Whenever the cage door is open, even to change water, get a level headed spotter to help and for safety.
>>how much do they normal weigh as adults.
I don't know. I've heard 45 lbs, but I'm sure it really really varies and goes higher for many adults.
Frankly, if you're leaning that way anyway, a rtb might be easier. Hope this helps.
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0.1 chickens (Condor)
0.2 dog mutts (half ownership, only mine when they misbehave, Lucy & Amy)
0.1 Halflinger horse (Crissy)
0.1 Normal phase California Kingsnake (Sophia)
1.1 parents
Still searching for 1.0 WC human