Continuing the discussion from Introduce yourself!:
When working with sketchup most tools will respond to direct keyboard input.
This is most often done by starting an action and simply typing what you want.
So start to draw a line by click and release the start point, move the cursor in the desired direction, take your hand off the mouse and type in mid air, 8’6" and hit enter. The line will snap to that length.
This works with rectangles, start the shape and type 9’,6’ and hit enter and your rectangle will snap to those dimensions.
There are many other times you can type things, scaling, arraying, copying, specific locations…
The thing to remember is, don’t go looking for the box to click in, start the action and just type what you want and hit enter.