Depends on what you install. If you really want you can get a Debian install down under 200M, Ubuntu is based on Debian so I would assume it would be close. For default installs I would assume that Ubuntu will be larger, probably quite a bit too since there's a lot more functionality out of the box. And it's not really possible to get an Ubuntu install to be functionally equal to Windows because it would require removing a decent amount of essential functionality.