macfidelity

I guess the most known command for basic disk-size informations is df or df -h (for human readable output) but there are better solutions. df itself is supposed to be used for displaying free disk space but not for a quick overview about real hd-sizes.

You can easily combine fdisk with a grep and the result should give you more relevant informations about your harddisk then just a stupid df

How to:

fdisk -l | grep Disk