You're going to have to do a lot of educated guessing because there's no hard and fast source for this kind of information. I searched around and found patches of information here and there, like for example that Forrester Research said Americans spent $18.3 billion online for apparel, accessories and footwear in 2006, and Forrester was expecting that to go to $22.1 billion in 2007. That came from a google search for "online clothing sales statistics."
You might also try to find out how much sales online are for some of the major publicly traded companies, like Gap for example, and see if you can find out how much they've grown.
You're going to end up finding related pieces of information and trying to put them together. And remember that what you're really interested in isn't just general market statistics, but a realistic way to estimate how much traffic your online site can attract and how much of that traffic will buy something. You're not going to find that in general market statistics.
Make sure to study successful online clothing businesses. Read their blogs, if they have them. Try to find out what makes them go. How much traffic do they get, and how do they generate traffic? How big are their sales?
I searched around and found http://www.plunkettresearch.com, which seems to do statistics and trends on the apparel industry in general, but nothing specifically online.
For more on methods and how to gather this kind of information, try the "Know Your Market" chapter at the (free online) www.hurdlebook.com; and the Supporting Information section of my plan-as-you-go book, which is also free online.