Hi Doodle and a very warm welcome to the forum.
So have you created a partition? Most Linux distros contain a partitioning tool.
I assume you are trying to create a dual boot with Windows if so see tutorial here ; Dual Boot Windows 7 and Ubuntu
If you have already created a partition it is too big. Given that you can read and write to Windows, everything you create in Linux can be stored in Windows. So you don't really need more that 30 or 40GB for Linux.
Remember that Linux uses a different file system , so Windows literally cannot see it.
There are 3 tutorials about partitioning on this page ; Tutorials - Windows 7 Forums
It would help if you went to your user CP and filled in your specs. Also let us know what Linux distro you are trying. I would recommend Linux Mint as it is very simple to use and comes with proprietary drivers etc
Hope this helps, John