/For=ForVolumeSpec /On=OnVolumeSpec /MaxSize=MaxSizeSpec - Resizes the maximum size for a shadow copy storage association ...

%1 %2 /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.  As certain shadow copies are deleted, the shadow copy storage 
    space will then shrink.  If MaxSizeSpec is set to the value UNBOUNDED, the shadow copy 
    storage space will be unlimited.  MaxSizeSpec can be specified in bytes or as a 
    percentage of the ForVolumeSpec storage volume.  For byte level specification, 
    MaxSizeSpec must be 300MB 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.  To specify MaxSizeSpec
    as percentage, use the %% character as the suffix to the numeric value.  If a suffix is not 
    supplied, MaxSizeSpec is in bytes.
    
    Example Usage:  vssadmin Resize ShadowStorage /For=C: /On=D: /MaxSize=900MB
                    vssadmin Resize ShadowStorage /For=C: /On=D: /MaxSize=UNBOUNDED
                    vssadmin Resize ShadowStorage /For=C: /On=C: /MaxSize=20%%