It's $50 (Canadian) for all sectors one-way(assuming you have booked through Air Canada direct). Dimensions for the box are listed on their site, but I'm pretty sure that it cannot exceed 32kg. Be mindful that it counts as one of your 2 pieces of luggage so you'll need to watch your limits as the excess baggage costs are quite expensive. If you want further clarification, call Air Canada direct on 1300 655 767 - they are extremely helpful!:)
I flew over with them last year, prior to flying you have to call up and let them know your bringing a bike, because they only allow space for a certain amount of bike boxes each flight.
Its $50 Canadian to get your bike on board. It cost me $150 all up because i filled my box right upto almost 32kgs, seen as though it counts for 1 bag of luggage I figured I might aswell.
It can be a hardcase or cardboard bike box, I took the cardboard version that you can buy from the Qantas desk at most airports. Cost me $16. Make sure to tape the box up a bit with ductape, give it a bit more strength in the handles and the bottom etc so it lasts a little while..