I'm afraid the the Sidewalks in the Curb library don't have exported parameters to adjust the width. The sidewalk libraries do however.
If you'd like to ammend the existing curb libraries so that you can scale the sidewalk you could try the following.
1 - Open the style editor, locate the generator and segments for the sidewalk (not curb stones) and add a transform operator for each input used in the generator.
2 - Export the Y Fixed Size
property for each generator and wire this to a Numeric
Node. You can use this to adjust the width from the modify panel.
3 - Set Alignment > Y
4 - Finally, adjust the generator's Y Offset
so that the pavements butt up to the curb stones correctly.
I've attached a sample amended style. I hope that helps, please let me know if you have any further questions.