Are you choosing Calculated Shipping for either the Domestic or for the International shipping? Silly question, but we all overlook minor details when we are frustrated.
Scroll down below the part of the Sell Your Item form where you enter the destinations and the service. You should find the place to enter weight and dimensions there.
And again, if you don’t already, use metric measurements. Canada Post went metric in 1974 and although some older Canadians still use Imperial measurements, they are never accurate. The change from Imperial to metric is found in a dropdown menu call Options with a blue question mark.