Varies a lot...depends on the length tanto you're going to do and how you want it to look.
What kind of look are you going for? Short/long, sleek/strong?
Let's say you are dealing with a short blade, maybe 7 inches. If you want a sleeker looking piece, probably go for 7/8" motohaba (width at machi) or just a bit narrower. Remember though, that the width is measured at the machi/shoulder at the base of the blade, and that it is probable that the blade has gradual profile taper all along its length. So the "width" is not going to be constant throughout the length of the piece.
If you want a good healthy width, you probably wanna go for about 1" wide at the same length.
I wouldn't suggest going much wider than 1" for that length, and preferably not more than 1.25" for longer pieces (11-12 inches)
So what kind of shape are you going for? Or just curious?