The Air Canada rules only permit a 16cm thick bag which is not a lot. Ryanair gives 20cm, United gives 22cm... how big is the sizer?
At 0:05 in this video, Measuring up Air Canada's hard line on carry-on luggage, you can see the sizer dimensions.
The Personal Item is 16cm depth.
At 0:20, you see several close-up shots of the sizer itself.