The PowerEdge 6850 uses SCSI hard drives and the largest SCSI hard drives that are available for the 6850 are 300GB. Those can be found at the link below and that includes the hard drive carriers as well.
6850 shipped in either a SCSI version or a SAS version. Assuming you have the SAS version and are using the PERC 5, you should be able to get up to 2TB drives. How many are you planning to get, what size, and in what configuration? What OS will you install?
Keep in mind a few things:
You cannot have a boot volume larger than 2TB and you cannot have a data partition larger than 2TB without first converting that "disk" to GPT.
Let's say you get five 2TB drives:
Your only real options are:
I would also recommend to you (if this will house critical services or data) that you not be tempted to purchase the cheap Desktop/Consumer-class drives and get Enterprise drives certified for use on a PERC controller (Dell co-branded, although you don't have to buy them from Dell).
If you plan to do this move to larger drives by replacing drives one at a time, let me know, as there are things you will need to know before doing so.