Open your visualisation and go to the Chart Editor.

Click "Advanced layout options".

In "Vertical resizing" you can choose either "fixed" if you want the height of the bar to resize depending on how much space there is. "Hug" means that the bars will always have the same height, no matter where you embed it. In this case, a scroll bar is added if there is not enough space to display the entire visualisation.

Select or type in the height you want the bars to be in pixels.

Don't forget to save your changes.

You have successfully changed the height of the bars in your barchart.