Extends or increases the size of an existing logical
             unit. To specify a new size for the unit, use the /SIZE
             qualifier. The size specified must be greater than the
             current unit size. When extending a unit with other units
             present on the same set of drives, some units may be
             moved (undergo volume expansion) to make space for the
             additional size required.

             Note that even though the UNIT size increases on
             SET UNIT/EXTEND command completing successfully, the
             increased size will reflect in OpenVMS only after the
             appropriate Dynamic Volume Expansion (DVE) steps such
             as SET VOLUME/LIMIT and SET VOLUME/SIZE are executed
             successfully at the DCL prompt. For more details, refer
             to Chapter 9 on Dynamic Volume Expansion (DVE) in
             "VSI OpenVMS System Manager's Volume 1: Essentials manual".

             Note: The /SIZE qualifier should be used only in conjunction
                   with the /EXTEND qualifier.

             Format:
                     SET UNIT <Unit_n> /EXTEND/SIZE=S

                     where Unit_n = (0-31) s=size in MB or GB (ex:10MB/20GB)