I would say that according to the current feature request, Symantec Drive Encryption is unable to encrypt partitions or drives that are 2TB or larger, which is why their workaround is to create multiple 1.8 TB or less partitions.
If your 6TB drive needs to be one single volume and you need it encrypted and cannot wait until Symantec does address this issue (feature requests may or may not be accepted), you will unfortunately need to look for a different encryption product.
If you wish to try and get this FR accepted and implemented ASAP, then you will need to contact your Sales rep so that he can escalate the request internally after creating a husiness case which includes how many licenses of the product you have bought, how many other Symantec product licenses you have bought, and if this lack of functionality will delay a current project (include costs incurred for this delay, etc).