A profile variable added to your community will begin to sync data beginning with the month it was added. As the Community Size report provides a snapshot of this data monthly, you will begin to see data for your profile variable starting with the month it was added.
Example
You imported a new variable in the month of October 2015. Filtering the Community Size Report by this variable does not display any data in September or before. The synced data will begin to display in October 2015.
More Information
Read this article to learn more about Community Size.