The scale of the image determines how much of the landscape can be seen in one image. If you choose a large scale of 1:300, the features on the image will appear much larger than they will on an image with a smaller scale of 1:1,500. On an image at a scale of 1:300, 1 unit of length on the image is equal to 300 of those units on the ground. For example, 1 inch = 300 inches. If you are not sure what scale would be best for your needs, you can order prints of different scales and look at the differences before deciding what you need.

Scale depends on the altitude of the airplane/sensor and the focal length of the camera. The formula for calculating scale is:

RF = 1/(H/f)

where RF (representative fraction) = the denominator of the scale (i.e. "300" from 1:300), H = the altitude of the plane or sensor, and f = the focal length of the camera.

Due to recent developments in remote sensing technology, the terms "scale" and "spatial resolution" are often confused. Just remember that "spatial resolution" refers to the size of the smallest resolution element that can be acquired by a digital sensor. For example, the spatial resolution of the Landsat TM sensor is 30 m x 30 m. Scale refers to the relationship between distances between objects on an image compared to actual distances on the ground.

Source:

Slater, Philip N. Remote Sensing: Optics and Optical Systems. Massachusetts: Addison-Wesley Publishing Company, 1980.