AutoCAD 2015 includes a new feature for viewports in model space. There are three clickable buttons as you hover over the upper-left corner of a model space which provide selectable options: viewport controls, view controls, and visual style controls. Once you select a viewport configuration, it is now possible to resize the viewport by clicking and dragging to resize as you wish. You can also click and drag over the plus sign at the top of the viewport, and it will create a new viewport as you drag across an existing viewport.
Notes from Cadalyst tip reviewer R.K. McSwain: Levi is correct: In AutoCAD 2015 you can resize and create tiled viewports in model space much more easily than in previous versions, and by using nothing more than your mouse. The plus sign Levi refers to in order to create a new viewport is very tiny, but you'll know you have correctly selected it when you start dragging and you see the green viewport separator.