Final answer:
The most appropriate phrase to use when discussing clothing size with a customer is option 2: Let me measure you to be sure.
Step-by-step explanation:
A phrase that might be appropriate when discussing clothing size with your customer is:
Option 2: Let me measure you to be sure.
This option is the most appropriate because it allows you to accurately determine the customer's clothing size by taking their measurements. This ensures a better fit and customer satisfaction.
Using option 1 or option 3 may result in an inaccurate assessment of the customer's size, while option 4 puts the responsibility of determining the size on the customer, which may not be ideal.