container design questions

[eshbach]

what is the ideal height/diameter of a the tube?

can a different metal (like iron) be used for the tube so long as the base is copper?

any other advice for someone delving into this for the first time?

[blinky]

its hard to join disimilar metals iron to copper

[lalPOOO]

Iron would be a pretty bad choice for the tube due to rust from condensation. It might also not braze (if it does at all?) with the copper base.
You'd be better off using brass if you can't use copper I think. Any reason why you can't buy copper tube at your local hardware store?