[JKT]

Set up the page with the Keep fit to window setting selected and save it as a template. Pick that template as the default. All newly created documents will start with the window zoomed to fit.

However, new canvasses in the same document won't pick up this setting and you'll still have to zoom them. Very annoying - use the Send Feedback from the OmniGraffle help menu to ask that this feature be corrected.