Fix width of bar ticks (or axis length?)

I have a number of bar charts with varying numbers of bars - some may have a total of 15 bars, others 30. I am trying to get the size of the bars consistent across all the different charts, so a bar in a 15 bar chart is exactly the same width as a bar in a 30 bar chart.

I’m able to control the width of the bar relative to its tick but what I supposed what I really want do to do is set the width of the tick. I imagine this means setting the axis length to be (fixed tick size * number of ticks) and then adjusting the overall plot size to accommodate that - rather than having the size of the axis autofit based on the size of the plot.

Does anyone have any ideas? Thanks in advance