The https://github.com/HarryStevens/d3-geo-scale-bar library kind of hard-coded the baseline to 80px (in turn the size). In the logic, he uses it to calculate scale from projection and zoom.
Since the scale/size of the scale-bar is tied to the scale/size of the map, we can't increase the size blindly. In future, may be, we can implement our own scale function.