It means the exact amount of something, the "sweet spot", and usually that something is non-negotiable.

The phrase means "exactly" or "precisely" and refers to what was just said. It carries the connotation that one believes a request or demand is reasonable. For instance, a teacher may say to his students, "I don't expect you all to be straight-A students. I don't expect that you will all love math or enjoy it. I expect only that you each try your hardest at all times—nothing more, nothing less."

It usually means something like, "I will not compromise on my position." e.g. "After the divorce, I want half of all my partner's wealth as well as custody of our children: nothing more nothing less."

It depends, what is the context? It could mean the thing you're talking about is good the way it is, no need to change it.