What command do I use to find physical disk size?

You can try lsblk.

That should list all disks including size. It will also list partitions and mount points.

You could try parted

sudo parted -l

or if you know the particular disk's block device e.g. /dev/sda

sudo parted /dev/sda print 

As well as looking at the formatted partition size(s), take note of the 'Partition Table' line - if the disk was formatted with 'Partition Table: msdos' then you may be bumping up against the intrinsic 2TB limit - the disc may need to be converted to a GPT partition table in order to access the full 3TB

You can also try the following command:

sudo hdparm -I /dev/your_device |grep GB

Change your_device with a the device name (sda, sdb, etc)