What Medium Size should be specified when formatting a tape medium?
Eventually, it doesn't matter at all. This specified size will be used to indicate how much space is left on the tape. However, for reasons already explained in another Support Tip (Support Tip: Why is DP not using the full capacity of my LTO tape?) the total amount of data that will fit on the medium is not predictable. This means that a size can be estimated (based on experience) and specified at format time as an attempt to get a good indication of the free space on the medium. But in any case, the tape will be used by DP until it is full (or only until the end of the session in case of a non-appendable pool policy). And once it is full, the medium size will be reset automatically to the real amount of data that was written to it which will then hopefully be an even better prediction for the future. Please note that once the tape is overwritten with other data, it will again be impossible to predict how much will fit on the tape this time and so the total tape size may be a very good estimation, but will almost never be 100% correct.