The Windows 10 preview has been out for several months, but it still doesn’t work well in VirtualBox: the VirtualBox graphics driver crashes with “Windows cannot initialize the device driver for this hardware. (Code 37)” or “Windows has stopped this device because it has reported problems. (Code 43)”. This means you cannot use “auto-resize Guest Display”. Fortunately, I saw in 4sysops.com this nice work around. In your host machine, add a custom video mode for the monitor size you will use in fullscreen mode. While VirtualBox is not running, execute the command “/cygdrive/c/Program\ Files/Oracle/VirtualBox/VBoxManage.exe setextradata vs2013-win10 CustomVideoMode1 1440x900x32“, where “vs2013-win10” is the name of the virtual machine, and “1440x900x32” is the screen resolution of the monitor you plan on using. Then, start VirtualBox, and log into Windows 10. Right click on the desktop, select “screen resolution” for the monitor you want to use, then enter fullscreen mode.
As you use Windows 10, “Windows Update” will be performed periodically. Occasionally, the updates, which typically involves most files of the OS during this pre-release period, may not install due to disk space of your VDI disk image. That can be fixed using /cygdrive/c/Program\ Files/Oracle/VirtualBox/VBoxManage.exe modifyhd YOUR_HARD_DISK.vdi –resize SIZE_IN_MB.
Note: After extending the size of a VDI using VBoxManager, you will need to extend the partition within the VirtualBox guest OS so that the guest uses the extra space. For example, for a Windows 10 NTFS partition, go to File Explorer | This PC / right-click | Manage -> Disk Management -> left-click the partition -> Extend Volume.