The blank must be slightly larger than you want the finished piece to be.
I don't think there is a 'standard' stopper blank (or for that matter, a standard blank for anything). In the case of coffee tampers, the notion of 'standard' is even more illusive because there are so few manufacturers who sell the metal bottom for coffee tampers. I made myself a coffee tamper back when I first got started in wood turning, and my search only turned up two suppliers - one outside Seattle, and the other in BC (Canada). Curious that they were so close to each other - guess it has to do with the fact that the folks in those parts are particular about their coffee. But both offered tamper bases with a 3/8"x16tpi thread which happens to match popular wine stopper mandrels.. But the key dimension was the diameter of the flat top - where the metal meets the turning. As I recall, it was a relatively simple matter of measuring that dimension, and then turning to match.
And by the way, there is no rule that says the tamper must have a metal base. You could make the whole thing from wood if that's your preference.
