Here's how you can change the size of bars in a chart in Excel:

1. First, select the chart that you want to modify by clicking on it.
2. Next, click on the bars that you want to resize. You should see small boxes appear around the edges of the bars.
3. Once you have selected the bars, right-click on one of them and choose "Format Data Series" from the drop-down menu.
4. In the Format Data Series pane that appears on the right-hand side of the screen, you should see a section called "Series Options". Here, you can adjust the width of the bars by using the "Gap Width" slider.
5. Move the slider to the right to increase the width of the bars, or to the left to decrease it. You can also enter a specific value in the box next to the slider if you want to be more precise.
6. Once you have adjusted the width of the bars to your liking, click "Close" to apply the changes.

That's it! Your bars should now be resized to the width that you specified.