I don't read the manual when I start using another, I just jump in and check the help file if I have an issue.
Your long string of questions makes that obvious. You could go to learn.sketchup.com and go through the SketchUp fundamentals. You could also spend some time going through the Square One videos
You must display the subtitles to watch the tutorial in English or Spanish.
This tutorial, a little old but still valid, explains how to give dimensions with the push pull tool and also how to resize lengths.