[Beginner] GUI Surrounding the 3D Scene

I am making a first person dungeon crawler similar to Dungeon Master, Wizardry or Legend of Grimrock.
I want to resize the 3D view to about 3/4 of the screen and then have the GUI on the bottom and the right side.
Kind of like this (sorry for the awful drawing):



How would I go about this? Thank you in advance!

seiesos

***UPDATE***:

I have resized the view port of my player camera, that's working well.
My problem is, that when I create a UI Panel, the position of it in the Game View is completely different to the position of it in Play Mode, and different again when I actually run the game.
How do I get the right sizing and positioning so that it fits exactly the area outside of the camera view port? My screen resolution is 1600x900.

Canvas settings:


Play Mode:


Running the build:

 

Your aspect ratio is changing when the window size does.

To fix your Game-view aspect ratio:
- Click "Free Aspect" at the top of the window, add a new setting with your 1600x900 resolution

To fix in your build:
- Go to your player settings. Under "Resolution and Presentation" uncheck "default is native resolution" and set the default resolution to 1600x900
- Also in your player settings under "Resolution and Presentation", find "supported aspect ratios" and disable all of them except 16:9

If you would like your UI to resize automatically how you want it to when the aspect ratio changes, you should look up some UI tutorials. It's fairly complicated, but you can achieve fantastic results. This tutorial goes into complex UI scaling around the 5 minute mark. https://unity3d.com/learn/tutorials/modules/beginner/ui/rect-transform?playlist=17111

 

Thank you!