OG makes a new page using its default template. If you open the Template Chooser (File menu) you can see the templates that OG provides and any that you've created. A template defines the window size, among other things. As I recall, OG's default deafult template does not have a full-size window.
In the template chooser, click New Template and and then set it up the way you'd like to see new pages in general. You can set window size, scale, canvas size, diagram layout, graphical elements, whatever. Save it with an appropriate name. (It will be saved to OGs template folder in ~Library/Application/Support/The Omni Group.)
Once you've got your new template in the chooser, select it there and click "Set as Default."