The audiences will commonly show TOO_SMALL until the job which calculates whether the minimum number of recent users (as per our user privacy guidelines) has finished running, and this can take upwards of 24 to 48 hours in worst case scenario (any longer and you would be fine to raise the issue again with these details included).
Basically you can think of setting up audiences on Real Time Audience as a two step process, the first step is to seed the audience with at least 500 users (to protect our user’s privacy). The second step, which happens in near-realtime, would be to add or remove users from that audience once it has reached a calculated min-size.
