/Provider=ProviderNameOrID /For=ForVolumeSpec /On=OnVolumeSpec /MaxSize=MaxSizeSpec - Resizes the maximum size for a shadow ...

%1 %2 [/Provider=ProviderNameOrID] /For=ForVolumeSpec 
    /On=OnVolumeSpec /MaxSize=MaxSizeSpec 
    - Resizes the maximum size for a shadow copy storage association between 
    ForVolumeSpec and OnVolumeSpec.  Resizing the storage association may 
    cause shadow copies to disappear.  If MaxSizeSpec is set to the value
    UNBOUNDED, the shadow copy storage space will be unlimited.  As certain shadow 
    copies are deleted, the shadow copy storage space will then shrink.  MaxSizeSpec 
    can be specified in bytes or as a percentage of the ForVolumeSpec storage volume.  
    For byte level specification, MaxSizeSpec must be 1MB or greater and accepts the 
    following suffixes: KB, MB, GB, TB, PB and EB.  Also, B, K, M, G, T, P, and E 
    are acceptable suffixes.  If a suffix is not supplied, MaxSizeSpec is in bytes.  
    To specify MaxSizeSpec as percentage, use the %% character as the suffix to the 
    numeric value.  If the provider is the supplied Microsoft provider, MaxSizeSpec 
    must be 100MB or greater.
    - The Provider ID can be obtained by using the List Providers command.
    When entering a Provider ID, it must be in the following format:
    where the X's are hexadecimal characters.  
    Example Usage:  vssadmin Resize ShadowStorage 
                            /For=C: /On=D: /MaxSize=900MB