The windows have been sized such that the pilot can see what is going on in the scene without taking up a ton of real estate on the screen.
Screen size is all about priorities. Once you are in the air we agree that seeing where you are going is more important that seeing the Terrain Awareness plot. We suggest hiding it when flying. You can tap the Terrain Aware button while flying to see where you are currently.
There is not currently a full screen option but we may be offering a full screen mode in the future. The issue is the preview that is provided via the DJI SDK is actually pretty small and scaling it up to full screen makes it look pretty bad. The large size that is currently provided is near the full resolution that is actually provided.