nodeWidth property

The pixel width of each node in a, or the height in case the chart is inverted. For tree graphs, the node width is only applied if the marker symbol is rect, otherwise the marker sizing options apply.

Can be a number or a percentage string, or auto. If auto, 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.treegraph.nodeWidth