I have an extra in my garage so I measured it for you. It measures 19mm(ID) x 23mm(OD). Part number is ERR6434.
I tore a couple of them putting my truck back together three years ago this week when I did my headgaskets. So I bought two more and got it in successfully on the third attempt. Hence the spare. What made the difference for me was lubing it a little when inserting the pipe. Mike clearly has better technique than I.