Buying a used XBox- how can you tell what the HDD size is by looking?

The most definitive way to check is in software. Have them boot the Xbox, then:

• Scroll right to "System"
• Choose "Settings"
• Choose "Storage"
• Highlight the Hard Drive in the list
• Press "Y"

This will display the size of the hard drive in gigabytes.

If they can't (or won't) boot the Xbox, then run, run away as fast as you can. Failing that, some Xboxes are labeled with their hard drive size.

The "classic" Xbox 360 (the one on the left) generally has the hard drive size on a silver label on the drive's casing. The drive sticks out of the top (or left side, if it's laying down) of the unit. If unlabeled, it is probably 20GB. Note that it's possible to open the drive cage and swap the drive, so this is less reliable than checking it in the settings.

The newer models (the two on the right) don't appear to have any external indication of the size of the internal storage. You'll have to remove the side flap and check the drive itself, as per this Microsoft support document.. This is more involved than just looking at the top of the unit, but whether that is more or less complex than turning the Xbox on and checking is a matter of debate.