df -h would provide you with the size of the mounted partitions. If you do a
hdparm -I <device> (device could be /dev/hda or something similar) you will get a lot of info, but important in this case is the Configuration section, where you can find the physical capacity of the drive.
Code:
Configuration:
Logical max current
cylinders 16383 4047
heads 16 16
sectors/track 63 255
--
CHS current addressable sectors: 16511760
LBA user addressable sectors: 90069840
device size with M = 1024*1024: 43979 MBytes
device size with M = 1000*1000: 46115 MBytes (46 GB)
Håkan