In Webmin 1.590 I can see or create Partition bigger than 2 TB. After Upgrate Webmin to 1.600 no GPT Partitions are shown. It's also impossible to create any partition on blockdevices (in my case a RAID 5 array) which are > 2 TB. I think there is a problem with gparted.
Testet Systems: CentOS 6.0 and CentOS 6.3
After downgrade to 1.590 everything is doing well with working with the partition tool.