Hearthpwn

It says (wherever it is). in C'Thun buff cards' text. If I play Disciple of C'Thun when I didn't put my deck C'Thun  then play Unstable Portal and get the C'Thun it will be 6/6 or 8/8?

6/6 because it doesn't exist in your deck yet.

Unless its in your hand, deck or battlefield it won't buff. Not sure if it buffs when stolen.  

C'thun buffs in decks without like the Evolve C'thun deck that was floating around.  It didn't run C'thun but his buffs made the Emperor's trigger.

Not only will it be 8/8, even if you don't get C'Thun and you do get it up to a 10/10 Twin Emperor Vek'lor or similar will trigger.

http://www.hearthpwn.com/forums/hearthstone-general/card-discussion/138199-is-it-possible-to-buff-cthun-when-its-not-even-in

http://www.hearthpwn.com/forums/hearthstone-general/general-discussion/134498-question-do-i-need-cthun-to-activate-other-cthun

http://www.hearthpwn.com/forums/hearthstone-general/general-discussion/122233-dose-cthun-have-to-be-in-your-deck

http://www.hearthpwn.com/forums/hearthstone-general/card-discussion/156408-cthun-question-does-it-need-to-be-in-your-deck

http://www.hearthpwn.com/forums/hearthstone-general/card-discussion/139855-are-cthun-buffs-bugged