VMS Help EXCHANGE, INITIALIZE, Qualifiers, /ALLOCATION  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /ALLOCATION=n

    Specifies the allocation of a new virtual device file in terms
    of 512-byte blocks. The allocation specified is the number you
    entered as n. If you do not specify the /ALLOCATION qualifier
    when you create a new virtual device file, the default allocation
    is 494 blocks, the size of a single-density diskette. The maximum
    allocation is 65,536 blocks.

    A virtual device file is usually the size of a standard device
    supported by both RT-11 and OpenVMS. These sizes are as follows:

    Device    Blocks

    TU58      512
    RX01      494
    RX02      494 (single density)
    RX50      800
    RX02      988 (double density)
    RX33      2400
    RL02      20480
    RK06      27126
    RK07      53790

    You can also use the /ALLOCATION qualifier to reduce the size
    of a physical device. For example, if you want to prepare
    an RL02 disk but have only an RK07 device available, you can
    initialize the RK07 to a volume of 20,480 blocks. When the RL02
    is available, you can transfer the files to the RL02 knowing they
    will fit on the smaller device.