Hi Brian, thank you for contributing to the 8x8 Community.
This link provides information on how long we can store calls. https://docs.8x8.com/8x8WebHelp/8x8-storage-and-retrieval/Content/StorageAndRetrieval/StorageTypes.htm
There is an option to bulk download from Admin Console.
Unfortunately, we won't be able to provide details on how much space your calls would take, that depends on the number of calls and their length.
Please let us know if you need more information.