I remember the novel was Japanese, and the main character found a damaged war Mech in the forest near his village left over from a previous war. He spent years repairing it little by little. Then a mercenary unit with a Mech of their own passes through, and he ends up in a fight with it, using his just recently repaired Mech. He then accepts a recommendation I think, to the military academy in the Capital, and joins the pilot program there, which is very competitive.
Well, I usually think of Mecha as being exclusively Real Robots, and once you get into much over 15m you're usually into either Giant Robots or you're dealing with non-terrestrial models and that's a whole 'nother discussion.
For me, Mecha implies piloted mecha of some form & shape between 1 and a few crew members ( some types have more). Mech implies a humanoid shaped smart robot, at most a few sizes above humans. And robots implies anything that has some type of artificial or virtual intelligence to help control it, but can otherwise be exactly like a mecha with the pilot part. Basically Mecha : Piloted Mech : AI / VI / other intelligence controlled Robot : AI / VI & possible pilot But that's just my opinion right now.