Each workspace now has a limit on how many domains you can add and how much storage your uploaded assets can use — 5 domains and 1GB by default. If you try to add a domain or upload a file that would put you over the limit, you'll get a clear message telling you exactly how much room is left.
You can track usage at any time: the Settings page now shows a Domains and Storage usage section, and the Assets page shows current storage usage right above your files.