I don't think Canada should fully be in the EU, but some kind of set of bilateral arrangements like Switzerland has would be a good start.
Since Canada has lots of historical ties with the UK, it might work out well to have some kind of arrangement with the UK, Canada and the EU. That would let Britain start to undo some of the damage from Brexit, while also allowing Canada to reinforce those ties to the UK.
40 is three million more than Ukraine, both in between Spain and Poland. Having a couple more mid-sized countries would enable integration of Turkey which eeks out Germany in terms of population.
As the EU mostly does not decide population based but with equal votes per country that wouldn't be an issue.
The larger issue is the fact that the European and US regulations clash in a lot of things so it would be harder for Canadian companies to export to the US then.
Actual membership for Turkey hasn't been a serious option for a long time. Turkey knows it, EU knows it. There simply is no other label than "candidate" that could better describe what TR means to the EU. They want to keep it as close as possible, and TR benefits from it, a lot. A symbiosis, if you will. As for the Switzerland model, the EU clearly stated, multiple times, that they don't want to repeat the unholy mess of relations they created with CH and, in fact, would rather transform it to something in line with stock EU foreign policy.