You haven't stipulated if this is a desktop or notebook computer. If dual-booting on the same drive, you need to let Windows Disk Management create the free space. If what it declares to be free space, and is less than @Aravisian 's size advice, then I would think again, or consider buying an external drive. You also need to disable FastBoot and SecureBoot in the BIOS. You may also have to change power settings in Windows in respect of the hard drive, as from Windows 8 upwards, the hard drive never fully shut down to allow for speedier boot times, but also prevents booting from other systems. Additionally, we have no information regarding if you have Trusted Platform Management present (TPM 2.0). If so you will need to also follow this guide: