1.how big do they grow on average.
4-5 feet.. average adult weight is probably around 2-3kg for females maximum is about 5kg.
2.i have a dog and my mum and dad are saying if it escapes it could attack or strangle the dog :S i would never forgive myself if something happend.
They wont bother your dog. if you got an aggressive one by chance it may strike in defense but do no harm. i trust them close to my cat.. maybe if you had a pet rat you would have a problem


3.how much for absoplutly everything would they cost on average?
Depending on your setup i would budget at least $400. The expensive part is the thermostat and it is not wise to cheap out on that one..

4.what size tank do they need for a full adult?
They don't need alot of height a 2x3x1 would be enough. Many people including myself keep them in rubbermaids..