The rigid portion of the stock line is 1/2" ID, the flex portion is about the same.
When you buy hard line, steel, copper-nickel, stainless... they'll be listed by the Outside Diameter.
~1/2" ID is probably going to be 5/8" OD.
I think copper coiled tube is listed by the ID and has a larger OD... that's confusing, Not at all recommending you use copper.