The 'scale of features' slider in the design map parameters wizard determines the height and width of features in the design map, such as valleys and hills. With a large feature scale setting, your map will feature taller and wider mountain ranges, whereas with a smaller feature scale setting, you map will consist of smaller hills.
This is demonstrated below, where where I've made terrain with feature scale settings from 0% up to 100%:
Feature scale | Design map | Heightfield | 3D render |
---|---|---|---|
0% (left, tiny) | |||
25% | |||
50% (centre, default) | |||
75% | |||
100% (right, vast) |