I opened the preference plist & saw there was a key for 'startingWindowSize', or somesuch value. It also lists 2 dimensions IIRC. It's com.apple.iWork.Pages.plist in ~/Library/Preferences/ (~/Library is hidden - use the Finders 'Go menu > Go to folder…' to open it). Veiw in TextEdit or Xcode if you have it (xCode's plist viewer will show it neatly).
So I opened the app (from the apps folder - not via Open with).
The 'starting point pane' opens with the 'templates' (there may be a pref to disable this). I Changed the size of the window & quit, next time it opens it was remembered. The .plist also updated.
There doesn't seem to be any keys for the last document used, so unless Apple save the setting within the actual file I can't see where they will add the window position or size to the prefs.
There may be somewhere to add it into Pages/ Numbers recently used items, but I didn't look for a plist handling that, sorry.