How to determine the true size of a partition? (df != fdisk)

[elivings]

I'm using a Promise TX4 RAID card to create a four-disk array, made up of
two pairs of striped, mirrored 60G drives. In other words, I take two 60G
drives and stripe them together into a 120G partition for performance, then
mirror that striped pair with another identical to it for data integrity.

Anyway, fdisk correctly shows that the RAID disk has 120G available on it
(the sum of the two striped 60G discs), and that almost all of it is
partitioned into a root partition with a little left over for swap and boot.

However, df reports that the root partition only has 60G on it. It seems
like fdisk is correctly querying the pdcraid module and communicating with
the card to discover the striped 120G virtual disk, while df seems to be
physically querying one of the physical drives in the array and learning
it's only 60G in size.

I don't really care what df thinks if I've really got the full 120G that
fdisk and I think I have.

So, the key question is: am I going to run out of room at 60G or 120G? How
can I find this out (without trying it, which I'd like to avoid having to
do...). Is there some other utility that can tell me how much actual,
physical storage I have on the disk?

Thanks,
Eric