In D3 version 4.x, d3.scale.ordinal() has been changed to d3.scaleOrdinal and d3.rangeRoundBands has been changed to:
         
        
To find the width of the band in a band scale you have to use:
scale.bandwidth();
According to the API, bandwidth():
Returns the width of each band.
Here is a demo:
var scale = d3.scaleBand()
  .domain(["foo", "bar", "baz", "foobar", "foobaz"])
  .range([0, 100]);
  
console.log("The width of each band is " + scale.bandwidth() + " pixels")
As you can see, the bandwidth depends on the number of elements in the domain, the extent of the range and the paddings. Here is the same snippet above, with paddings:
var scale = d3.scaleBand()
  .domain(["foo", "bar", "baz", "foobar", "foobaz"])
  .range([0, 100])
  .paddingOuter(0.25)
  
console.log("The width of each band is " + scale.bandwidth() + " pixels")