Here are a few ways to measure the size of a portafilter: 1. Use a ruler - Place the portafilter on a flat surface and use a ruler to measure across the inside diameter from one side to the other. This will give you the size in millimeters or inches. 2. Compare to standard sizes - Portafilter sizes are commonly 49mm, 53mm, 58mm, etc. You can compare your portafilter visually to one of these standard sizes to get an estimate. 3. Use calipers - For the most precise measurement, use a pair of digital or dial calipers. Open the calipers to fit across the portafilter basket diameter and take the measurement. 4. Weigh the volume - Weigh the amount of water the portafilter basket holds when full to give you the volume capacity in mL or oz. Compare to standard volumes for different sizes. 5. Look up make/model specs - Check the specifications or product description for your espresso machine to see if it lists the portafilter size. This info is often provided by the manufacturer. The key is measuring across the inside diameter of the portafilter basket where the coffee grounds sit. This will determine what size tamper, baskets, and accessories are compatible with your machine's portafilter.