if you want to know the size  for admin task , you can know
from the DB but if you want it for the client , i dont know if there is any
webpart to do this .

Yes I am aware of where to get the information, but we are trying to
see if we can place a webpart  on the user's sites so they can see what
their current site  collection size  are.

This would be handy is there was just a webpart that said it, this is
why I am asking. We were going to develop one, but like I said in my
original post, if there is one out there already, then I would not
want to re-invent the wheel.