Welcome to Linux! We're happy you're here! I was a long-time Windows user until about a year-and-a-half ago, when I did what you're doing, partly out of a bit of frustration with Windows but largely because I was just curious. I figured I might as well see what it was like on the other side of the fence. Now I still have a Windows partition that I keep just because, but I almost never boot into it: I'm all Ubuntu now.
So to answer your questions, it looks like you have plenty of room on your HD. You won't use as much space with Linux as with Windows, but if you end up staying with Linux you'll want more space eventually where you can save pictures, music, etc., so I'd recommend going with about 30 GB (as kmassada also recommended). It'll give you plenty of room for messing around and learning - which is one of the fun things about Linux.
But you might want to keep some of the rest of the space clear for now, because you might get the desire along the way to install and test other flavors of Linux. Hey, they're free, right? Why not try them out? A free 20 or 30 GB of space would let you easily try something else out. (In my year-and-a-half, I've tried out lots and lots of distros, including all of the popular ones; I'm still with Ubuntu, but I'm glad I looked at the others, too.) So if you give about 30 GB to Ubuntu, you'll still have almost twice that space left for trying out other distros. (And when you're done testing one, if you want to get rid of it you can just delete the partition.)
As for the desktop environment, well that's pretty subjective. I really like Unity but I know others prefer different DEs. One good way to test some of the others is to install a distro that comes with another DE (try Kubuntu, for example, in another partition, to experience KDE, or Xubuntu or Lubuntu to try some of the lightweight DEs). You can easily try Cinnamon or Mint with Ubuntu, installing them and then logging into them. And you'll want to check out Gnome, too. As you read more and learn more about Linux, you'll encounter a lot of things about them all - and even a few irrational rants.