Where VSS settings are not accessible from the GUI, the following commands can be run from an elevated command prompt:

To show the current volume shadow copy storage associations:

vssadmin list shadowstorage

To resize the volume shadow copy storage:

vssadmin resize shadowstorage /on=X: /for=Y: /maxsize=Z

where

X = drive holding the shadow copy storage
Y = drive being shadow-copied (backed up)
Z = new shadow copy storage size along with the unit size (e.g. 10GB)

For example:

vssadmin resize shadowstorage /on=X: /for=Y: /maxsize=10GB

Run the vssadmin list shadowstorage command again to validate the new volume shadow copy size.

Note: The above commands only work if a shadow storage area has been added before. 

To add shadow storage area:

vssadmin add shadowstorage /for=Y: /on=X: /maxsize=Z

where

X = drive holding the shadow copy storage
Y = drive being shadow-copied (backed up)
Z = new shadow copy storage size along with the unit size (e.g. 20GB)

For example:

vssadmin add shadowstorage /for=d: /on=c: /maxsize=20GB