There is a thread on this, Top of my head i think they were 4mm I/D X 1mm section, I found it better to buy an O ring kit from ebay that contained the size and a lot more too, for just a couple of quid.
Just checked my old thread, Size mentioned above is correct, got an assorted kit of 225 pieces of O rings for £2-25p with free postage, all in Viton.
To get you out of trouble just take a larger o-ring of 1mm section and cut it with a sharp knife. Wrap it around a 4mm drill and cut it again, join with superglue. You can actually get kits for making o-rings this way.