There is a select box for this at the bottom left of the editor window – choose 100% 😉
I was unable to find a keyboard shortcut for it, though zooming in and out can be done using Ctrl + > and Ctrl + <.
Please note the horizontal scroll bar must be turned on to see the zoom level.
Tools / Options / Text Editor / All Languages / Scroll Bars
Another option (Visual Studio 2013/2015) is to use Ctrl with the mouse wheel (up to zoom in, down to zoom out).