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) |   |   |   |