Hello,
Thank you for sharing your concerns with us. To clarify this better, the user list size will show as zero until the list has at least 1,000 members for privacy purposes. After that, the size will be rounded to the two most significant digits. Also, it takes 6 to 12 hours for a list to be populated with members. Please refer to this guide for more information. Let us know if you still have any concerns.
Regards,
Nikisha Patel, Google Ads API Team