I actually had to provide the number to
resize2fs, it didn't work without. Try this:
$ resize2fs /dev/sda1 6TB
It will tell you what's the max size, replace
6TB with that number and it worked.
That said, you need
fdisk to resize the partition first.
Had that, too, recreating the instance with Ubuntu 17.10 gave me a GPT. Although not sure it's the OS choice or drive size that actually did that, since I initially created the first instance with just 20GB instead of 20TB...