Please check the below:
- How do I remove gaps between the bars?
- Adjust space between bars (have problem in grouping calculated field)
- Adjust space between bars (have problem in grouping calculated field)https://www.senturus.com/blog/tableau-tip-add-space-between-groups-of-bars-in-charts/
I'm sure there are more elegant methods of doing this, but the easiest way of moving the header is to duplicate the sheet and then hide everything but the title you want, then hide the title that you want moved to the bottom by r/clicking on it and hiding it using the menu option.
To change the size of a bar, simply click the size mark and drag the slider all the way to the right