Setting Virtual Machine Hard Drive Space
Click the Hard Disks icon. This is probably the most important step of setting up your new virtual machine. Select your Mac's hard drive from the left hand side of the window. In the right hand side of the window, you will see a slider allowing you to set the size of your virtual machines hard drive. Understand that whatever you adjust this setting to will take that amount of hard drive space from your Mac computer and give it to your virtual machine. For example, if you have a 160GB hard drive in your Mac, and set the slider to 60GB, you will have 100GB of usable space for OS X, and 60GB of usable space for your virtual machine (windows). OS X will still show up as having a 160GB hard drive, but anything you add to your virtual machine will diminish that value. Alternatively, if you check the hard drive usage in your virtual machine (windows) it will show up as having 60GB of space, total.
Also, understand that what you choose now for hard drive space is very difficult to change in the future. You cannot just slide this slider again anytime that you want more disk space. If you try to, the slider will slide, but Windows or any other virtual machine you may have will not recognize it.
Trying to add more hard drive space to your virtual machine in the future will require partitioning of your Mac's hard drive and transferring it to your virtual machine to use, which is not a simple process.
After you are done setting the hard drive space, make sure that there is a checkmark in the "Pre-Allocate Disk Space" box. Click Show All, and exit the window.