Dell may have sold the T20 with only up to 3TB drives. Any modern system supports drives larger than 2TB (no currently achievable limit exists on modern Intel SATA controllers). So, 2 5TB drives shouldn't be an issue. Do note that unless you add a raid controller, at best you might be able to run firmware raid (supported by Windows and maybe Linux, but not things like BSD or VMware or so).  If you were planning to run software raid, then obviously pretty much any OS should work with this.
Hi, I am going to build an MS SQL server for a small lab. I have chosen the Dell PowerEdge T20 server Spec1-T20-004 configuration. And I want to use two additional 5TB drives in RAID1. Since every document, i found from dell (including phone support) they only sell/support drives up to 3TB. I don't... 
Dell PowerEdge T20 : Hard Drives