Why not make a virtual machine (tools like VirtualBox/VMPlayer) Why do you need a dual boot? As for Windows 7 i will not recommend it to shirk your HDD. A safe way; backup your data (to cloud, nas, usbdrive etc) then perform a clean installation where you can select the HDD size. (20GB should be enough for windows 7 - ultimate)
Otherwise i have to agree to Neemobeer