You would use millimeters (mm) to measure the width of a penny and a dime. Here's why:

* Metric System: Millimeters are a unit of measurement within the metric system, which is widely used for everyday measurements.

* Small Size: Pennies and dimes are relatively small objects, making millimeters an appropriate unit for accurate measurement. Inches would be too large, and centimeters might be too coarse.

Here's a rough estimate of the widths of a penny and a dime in millimeters:

* Penny: Approximately 19.05 mm

* Dime: Approximately 17.91 mm

Keep in mind that these are estimates, and actual measurements may vary slightly depending on the specific coin.