To float the 3D viewport double click on the 3D viewport's name tab. You can also hold down the left mouse button over the name tab and drag it, the viewport will then float when you release the left mouse button.. You can then re-size and move the 3D viewport. You can even move it to another screen. If you save the design, then the next time the design is opened the floating 3D viewport will appear.
Floating 3D Viewport