Thanks for replying, yes I know how to change the dimensions, I was wondering how to change the default dimensions permanently, and what the current default is used for, what format.
According to Wikipedia 480 * 270 is a Web 480x HD. The size is hard coded I think. Probably can be changed and written to the synfig settings file. Regarding to Span it just enlarge the units of the canvas (zoom out) when increase its value. The resolution is not changed.
Kinda weird, yes. If you find more information please write it to the wiki. The only documentation we have about that is the source code.
-G