I'm pretty sure that brand-name Moby wraps are 23 inches wide by about 5.5 yards long. I made my own Moby-style wrap based on those dimensions, and I love it for the most part. For me, the length is perfect, but I'm a size 16, so you'll probably need quite a bit less. Also, the 23-inch width feels like a bit too much to me -- I think I'd be happier with 20 inches.
They're incredibly easy to make -- just buy 5.5 yards of stretchy cotton fabric and cut it length-wise to the width you want -- no hemming or anything required -- mine has held up through almost 6 months of heavy wear and lots of washes with no fraying or anything.
That way you could try it out at 23 inches x 5.5 yards and then cut it down if you feel like it's too big.
(One note if you decide to make your own -- this step would be optional, but it's handy to have some sort of marker at the center of the fabric so that you can line up the center with your belly button when you're putting it on. I embroidered a design in the center, but something as simple as a Sharpie mark or button would work if you don't sew.)