After talk in this thread I get enough encouragement to start diving in the code and understand how to remember preferences between synfig sessions.
The first result of this particular challenge is this patch. There you can see that the Settings dialog has been modified to allow user to set the dimensions of the new canvas by two numerical inputs.
Sorry, the rest of the dialog is in Spanish.
The question is: Which other properties for the new canvas command you consider should be remembered?
I plan to group all the settings for the canvas in a new tab called “Canvas”.