2TB.  Assuming (since you have a RAID 5) that you are using a PERC 5 or 6, 2TB is the largest disk they will recognize.  They also do NOT support non-RAID, so you must configure your single disk as a RAID 0 before it will be usable by the OS - this is done either in the CTRL-R utility during POST (be sure to Initialize) or from the OpenManage software in the OS.
I have a poweredge 2900, raid5, but I just want to add another HD not as a raid but an extended drive to host files. What is the largest HD I can use?
Additional HD