Thanks for using Syncfusion product.
We have analyzed your query. The box width of BoxAndWhisker series and bar width in Bar series can also be customized using ColumnFixedWidth and ColumnWidthMode properties. We have created a sample for BoxAndWhisker series in which we have customized the box width using ColumnFixedWidth and ColumnWidthMode properties, also width of bar in bar series can be customized using this property. Kindly find the code snippet,
this.chart.ColumnFixedWidth = 10;
this.chart.ColumnWidthMode = ChartColumnWidthMode.FixedWidthMode;
Screenshot for BoxAndWhisker series:
Screenshot for Bar series:
For your reference we have attached the sample. Kindly find the sample from the below link,
More details on series customization can be found from below link,
Let us know if you have any concern.