If you build one the way that John Snow described in the April issue of Outdoor Life, then the small end should be a little larger than an inch in diameter. You want it large enough to fit your slightly open mouth when you call, but not so large that air gets around it. I picked up a classic birchbark call last fall before a hunt in British Columbia and I just measured the small end: 1.3 inches. The guide on that hunt, incidentally, had a beat-up old birchbark call, but because the mouth end got splintered and cut his lips, he wrapped it with duct tape, more or less customizing the fit to his own lips. And reducing the number of cuts he gets in a full day of calling.

Hope that helps,
Andrew McKean
Hunting Editor