I selected 4x6 thermal labels in the settings but when I as to print the label it comes out as 8.5x11.
The 4x6 label size is only available to contract/commercial customers.
If you would like to become a contract/commercial customer, please contact Canada Post. When your account has been changed redo the signup process, and enter you contract number in the configuration. Then attempt to print out the 4x6 thermal labels.
PS: Please clear cache and reindex data after performing the above operation.