Aha!
I've found out how to do it!
Take a look at page 67 of the manual:
http://www.archos.com...al_archos_gen5_v3.pdf
The 'System Settings' screen, which is illustrated there, shows "Free Storage Space 5.6GB out of 28.4GB". That means that the illustration was taken from a 30GB model. (The size of the drive which is displayed by a computer is always slightly lower than the nominal size of the disk. If you're interested in the reasons, see this thread from last night:
http://www.theanswerb.../Question1027253.html ).
So, if you can get to the same screen on your Archos device, you'll be able to see the size of the hard drive.
Chris