How do I set a test size for A/B testing broadcast email?

Once you have drafted versions in the Message tab, you need to review the sending summary by first setting the test group size.

Step 1: Scrolling down the summary to the Set Test Size section, add the number of recipients that will be included in the test. Each version of the test will be sent to an equal number of recipients. Any remainder will be included in the winning version's recipient count. Please note that sending to fewer than 1,000 recipients may draw inconclusive test results; thus, we recommend a minimum test size of 1,000 recipients.