If you're using the VI client already then it's listed in the configuration->storage section when you click on the volume.
If you're still using VC1.x and ESX2.x then type vmkfstools -l vmhba0:0:x:1 at the service console.
Where x is your LUN number. (not always a 1 either, ls your /vmfs directory)
This will give an output like this:
Name: your volume name (public)
Capacity 1901273439744 (113321 file blocks * 16777216), 687865856 (41 blocks) avail
The 16777216 is the blocksize in bytes, in this case 16MB.
