Choosing the right External Catheter is critical in maintaining good health. Correct sizing of your Texas/Condom catheter is important and ensures comfort and reliability. Measure the circumference of the penis and match it to the closest catheter size offered. Sizes are usually shown in millimeters (mm) or inches (in) Tip: always use the smaller size available. An external catheter will stretch to provide a good fit but a loose external or condom catheter will not properly adhere.