If you are going to install operating systems on your computer (especially dual boot) then you really should understand partitioning and how to use the software for partitioning. There is a Mint Guide at the website and numerous tutorials on the Internet for this.
At any rate, you have reduced the size of the Windows partition by half. To get back to the original size you should use the Mint Live CD, then run "gparted" (Menu, Administration, Partition Editor), to delete the second partition and then increase the size of the Windows partition.
Personally, I would not use Wubi to install Mint if you really want to learn to use an operating system as it was meant to be used on its own partition. If I were you I would first of all read the Mint guide, learn how to partition and then install Mint properly onto its own partition.