Force: Similar to applying pressure when painting.The harder the brush, the less feathering of the edge, which will result in a more defined edge for your brush strokes. Hardness: Adjusts the hardness of your brush’s edge.Spacing: This will increase or decrease the distance between the brush marks in a stroke.
This will only be applicable if you’ve altered the Aspect Ratio or have an odd-shaped brush. To the right (positive) will make the brush shorter. Adjusting the slider to the left (negative) will result in a narrower brush.