The "optional download size is 0" error is rarely about actual disk space. It is almost always a issue. Start by verifying your Boundaries, ensure all content is 100% distributed, and try switching your download behavior from "Download all" to "Download as needed."
Shows if the client can find the content on the DPs. The "optional download size is 0" error is
If your Task Sequence includes Applications (rather than older Packages), the "Requirement" rules for those apps are evaluated before the download starts. If a requirement isn't met, the client might report a 0-byte available status. ensure all content is 100% distributed