Modify encrypted partition's size

Modifying the partition size of an SafeGuard encrypted volume is not tested and therfore not supported.

It's not possible at all.  I'm pretty sure you'd end up bricking the PC if you try to change the size.  Heck, resizing a partition on even an unencrypted drive is dangerous and could result in loss.  The safest, and really only way to go about it is to decrypt and uninstall Sophos, reboot, then do your resize and reinstall.

The following suggestions presume you have a sizeable external hard drive(s) or network share to use as the destination for backups, as well as quite a bit of time to devote to this task.  It also presumes you have confidence and experience in modifying hard drive partition tables and doing system maintenance tasks.

Before you do any partition resizing, get a good backup of the whole disk using a bare metal tool such as Clonezilla or RedoBackup.  (Any encrypted partition will be completely backed up, not just the space in use.) This is your CYA copy to get the computer back to exactly how it was before making any changes.  Note that, if there are bad sectors on the disk being backed up, Clonezilla is the better choice as it has the "rescue" option in Expert mode.

Next use a Windows application such as DriveImage XML to make a complete system backup from within Windows.  Then, once you're supremely confident the backup is good (say after restoring it to another hard drive and verifying) delete the encrypted partition and create a new one of the desired size, do a basic install of Windows on it and restore from the backup. 

Once everything is complete and you're satisified no data loss has occured, purge the backup done within Windows as it is not encrypted.  The bare metal backup, however, is safe to keep as it was done outside the encryption layer.

Hope this helps.