Why does my new Salesforce list show a list size of 0?

The Email Opt-out field must be visible in the Page Layouts for both Leads and Contacts. If not, any lists created will have a list size of 0. Additionally, all Leads and Contacts must have an email address in the Salesforce email field and any Lead or Contact with the email Opt-out box checked will automatically be excluded from new VR lists. Your Salesforce Administrator should review the following.

1. Log in to your Salesforce account
2. Go to Setup | Customize | Leads | Page Layouts and click Edit
3. In the top admin pane, locate Email Opt Out (under Fields) and add it to the page layout.
4. Repeat steps 2 and 3 with your Contacts page layout.
5. Try generating a list again.

 
If your list still compiles with a list size of 0, simplify your query settings to email only, this may point to a field with unparseable data in more complicated queries.

(during the current upgrade to TLS 1.1) Also check the "Critical Updates"  page, you should see an update labeled "Require TLS 1.1 or higher for HTTPS connections", click the deactivate link and try generating your list again.