If the icon for the Macintosh HD is visible on the desktop, click once to
highlight the icon but not open it; then use the File menu in Finder, to
click and look down the column and select Get Info. ~ This would give
you the information about the Hard Disk drive capacities.

And if your hard disk drive icon is hidden, you can use a Finder preference
selection to make it visible on the deskop, then use 'Get Info' on highlighted
icon. Or use ' Command + i ' to get info on highlighted icon file. (two keys)

Another way would be to use "about this Mac" and look into 'more info'
to open System Profile or System Information, depending on vintage OS.
The hardware information includes ATA drive specs, & other specs also
including software installed in the computer.
• Frederick (5 years ago) told of a way to Get Info -- if the HDD icon is present on desktop:
"To see how much room you have on your hard drive right click (Control key + Click)
and select Get Info from the drop down menu.and check Available"
This should work in most any Mac OS X version from early until present.
The screenshots above are from my G4 Mac Mini internal 93GB ATA HDD.
Good luck & happy trails! 🙂