A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
It doesn't seem that Excel lets you make the caps wider, but you can change them so they are more noticeable.
Click an error bar to select them all then use Command-1 to display the Format Error Bars dialog. In the left side click Line. On the right side choose Weights and Arrows. Choosing a different end style and size can make the error bar cap more noticeable.