The aircraft altitude, film format and the focal length of the lens will determine how much area is captured and the image resolution.

 
 

8km x 8km 16 meters

1km x 1km 4 meters

500m x 500m 2 meters

250m x 250m 1 meters

Image size vs. resolution is a trade off; even with a really long focal length, the area of the photo is determined by the altitude.  Flying too low will may not cover the area entirely and will increase the costs  and flying too high may not capture sufficient detail for the project. Defining a minimum mapping unit and accurately determining the project goals and required scales of analysis is vitally important for planning photo missions.