Can you take oversized luggage on Kingston to Toronto AirCanda?

As title says! We're coming from the UK and trying to book on oversized luggage flying through Toronto to Kingston (and return journey). We know that it is an Air Canada bus between Toronto and Kingston. We annoyingly must use a travel agent because its a work trip, and the travel agent keeps telling us we can't take oversized luggage on board. Yet the Air Canada website says that it will be possible. Can anyone advise? Can we just turn up with the lugagge?

I'll add: the travel agent really is just useless which is why we don't know whether or not to trust them.

(The oversized luggage fits within the max dimensions and weight of Air Canada)