I know that I can mount a Go-pro to the robot. But the can I put Go-pro exceeding the dimensions (18" by 18")?
A GoPro would be considered non-functional decoration as per R7-D. Any non-functional decoration would be considered part of the bot and must be fully within the 18^3 as per R4.
“<R4> At the beginning of any match, robots must be smaller than 18” x 18” x 18”.”
“<R7> Robots are allowed the following additional “non-VEX” components:
d. Teams may add non-functional decorations provided that these do not affect the robot performance in any significant way or affect the outcome of the match. These decorations must be in the spirit of the competition. Inspectors will have final say in what is considered “nonfunctional”.”
Hope this helped! I put my GoPro on our bot at our last competition and it gave really cool results!
Hope it works for you too! 
Nothing can exceed the 18^3 staring cube, nonfunctional or not. 
What about wires… If so I’m in trouble
Wires too, must be in the 18^3. Something to consider, no matter the size of your bot, if the item that extrudes furthest out of your bot is wires, you may want to reconsider that.
well, good thing refs don’t really care too much, me and a lot of teams would be in the spot. cant really control what a wire does without a million zipties 