Hi Jim,
"Interesting, I suppose you can spin them to size if you have and use a lathe."
Yes, there are only three sizes of pipe used in small steam engines: 1/16", 1/8" and 5/32" and I have found that the tightest these pipes can be bent without too much distortion is 5 times their diameter, in the case of the pipe that was bent 5/32" or in my terms 4mm is around a 20 mm profiled disc giving a 10 mm radius.
Take care Tony.