[CentOS] How do I check the real size of a filesystem?

Fri Jul 4 11:16:30 UTC 2008
Kevin Thorpe <kevin at pricetrak.com>

I'm having a spot of bother with a large (900GB) partition.

fdisk claims it's 900GB. I made an ext3 fs on it and df says it's only 
94GB. Which is correct? Or did I do something wrong?