I have defined a new custom audience (my first one) to make available as a remarketing list in my connected AdWords account. I see that my parameters show 1,450 active users that match the conditions within the past 7 days. When I save the audience, the text at the bottom of the audience summary reads:
Display audience size: 0
Search audience size: 0
Additionally, when I select this audience as the audience for a remarketing campaign in AdWords, it says there are "too few people on your remarketing list."
If you have only just created the list it will take some time to populate; whilst it forecasts how many people would have been added over the past 7 days, it won't add people retrospectively. Only people from this point on who match the criteria would be added. By the sounds of it your list should become active in a few days.