!2mm outer / 10mm inner from memory and a bit of guessing!
If you haven't got a set of vernier calipers to measure the outside of the steel pipe then use an open end spanner and see which one matches the size of the pipe, the pipe should slide neatly inside the jaws of the spanner of the appropriate size. This's only for a rough guide but it should give you the answer you need but beware the pipe might be an imperial size.
The size does look similar to the post above.
Looks like about 12mm or 1/2" OD.
Ultimately it's not critical, anywhere 10-12mm will be perfectly fine if you're trying to replace it.
Some modern cars it'd be as small as 8mm OD.
9.85 @ 145mph 202mph standing mile
https://www.youtube.com/watch?v=Ss_c7fML3rw
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