Sometimes I like to add posts just for the googlers... I know how it is to search the web over for info and can't find it. I noticed in my analytics today that someone landed on my page from google searching the above subject.
Here's how I do it....
For the length, I measure my DD from just below the collar bone down to the desired length (which for me is the knee), and add 1 1/2" seam allowance for the casing at the top and 1/2" seam allowance for the bottom hem.
For the width, I take the measurement of one of my daughters losely fitting shirts and add 3-5" depending on how puffy I want it to be.
I prefer more a pillowcase style top rather than a dress. They are still adorable and can be easily worn with pants. For that, I add about 3" onto the width of one of her shirts, for a dress I add about 5" to make it nice and full.
Here is the result.... (these are from this spring/summer)
I bought the fabric for my daughters pettiskirt from AFC-Express. The fabric used is 15 denier nylon chiffon. BTW I will be selling the pettiskirt on ebay. As much as I hate to, I really need the money. Consider it like a charity auction lol. See here for more info on why.
More for the googlers....
An EXCELLENT place to get info for making bows (bow making for dummies) is http://www.hipgirlclips.com/forums/. Awesome. Lots of tutorials on making bows and running a successful hairbow business.