Windows 10 has a built in image backup utility. It is accessed by opening Windows Control Panel and selecting "Backup and Restore (Windows 7)". Then choose "Create a system image". Follow the prompts.
Then create a system repair disk (available in the same menu) to boot the computer from. This will provide the tools you will need to restore a Windows image file, if necessary.
I used this method exclusively for several years to create full system images, but like fireberd says, I find Macrium Reflect much easier to use now.
An image file size can vary due to the number of allocated sectors that are copied. The free space will not be copied, so the image file can be smaller than the source drive full capacity.
I find that with the added image compression in Macrium, backup images are also generally quite a bit smaller that the native Windows images.
The only advantage that I still see with the Windows image version is that native Windows image files can be mounted directly as VHD (Virtual Hard Drives) where they can be browsed with Windows explorer. Then if you wish you can copy and paste any files/folders directly from the image, no 3rd party software required.
You can also do that with Macrium images, but Macrium must be installed on the PC in order to mount the proprietary Macrium image file format for browsing.
I still take one native Windows image once a month just as insurance in case that Macrium ever gets borked with an update at some point and I cannot get a good restore from it. That may be overkill, but I sleep better at night.