nodeWidth property

The pixel width of each node in a sankey diagram or dependency wheel, or the height in case the chart is inverted.

Can be a number or a percentage string.

Sankey series also support setting it to auto. With this setting, the nodes are sized to fill up the plot area in the longitudinal direction, regardless of the number of levels.

API Docs: https://api.highcharts.com/highcharts/series.dependencywheel.nodeWidth