Chris showed a few ways to do what you asked about, but in SketchUp there are alternate ways to do what you wanted to achieve. In this recording watch the lower right of the window. I draw a rectangle of any size, and type 20,53 Enter. I push pull (that’s the extrude like tool) up any amount, and type 48 Enter. Next I use the Move tool and raise one side up by any amount, and I typed 5 Enter.
All together it was only three mouse actions and three sets of typing.