You may need to create partition larger than about 3TB under GNU/Linux, with recent Hard Disk Drives having great capacity, with RAID configuration ...
Usually to create/manage partition with command line under GNU/Linux, fdisk is very adapted and powerful, but it is unable to create partition greater than 2/3TB (according to version, and architecture ?), because it needs to manage GPT partition (not supported by fdisk).
For that, you can use GNU Parted which is another powerful tool.
Instead of long explanation, this is a concrete example creating a 4TB partition on a RAID 10 (let's call it /dev/md16):
To begin, activate the GPT:
parted /dev/md126 mklabel gpt
Then, create a primary partition with all spaces (4TB):
parted /dev/md126 mkpart primary ext4 0 100%
Finally, formats it (for instance with ext4 and default block size):
mkfs.ext4 -v /dev/md126p1