Marshalls Shoe Megashop is indeed somewhat "mega." We almost drowned in our own drool. We are doing okay.
Unfortunately, you can't shop online, but if you click on "Take a Tour of the Megashop," you are taken to an interactive Quicktime video of a Marshall's store where you can zoom in on the shelves. We don't really know what you're supposed to do with this tool, but we totally played with it for a good two minutes.
Each shop is over 5,000 square feet and specially designed for awesome shoe shopping splendor. Check out your local Marshalls to see if you have a brand spanking new Shoe Megashop!