The trail is not as large as you may think ( can do it in day). I have visited but did not stay in the national park on the "left side" of the start of the trail. (Had no alcohol allowed on Canada day weekend, so kept driving). Also drove through another campground just shortly after that one, and it was close quarters field camping... Which in a tent, we weren't interested in. So drove to the about halfway point and stayed in the "hide away campground and oyster market" . Was decent wooded sites. Not sure about the large size that you need... We were tenting. Showers were gross and needed an update. This was about 15 years ago though.
On the "right" side of the trail is whycocomagh provincial park. This is a very large hill with lovely views. We really liked this campground. The staff was excellent helpful and friendly. Recently the province has pretty much removed staff from provincial parks, so I'm not sure if this place is still a great place to camp.
I suggest using good Sam online guide for more up to date info on Cabot trail camping.
Please use caution on the steep incline/declines. Engine breaking is strongly recommended.
Edit to add: if interested in cape Breton, I would strongly recommend heading over to Sydney and visiting the Fortress of Louisburg. This is not to be missed when in this area of Nova Scotia. This is the real deal and is so much better than Williamsburg for authenticity.
There is camping there, although I've only stayed in hotels in that area.