Still having trouble signing into my other account for the forum D':
First, can you clarify what you mean by 'detail'? (Do you mean description/imagery, or something else? And are they saying you're adding too little or too much?) And the second question...I think that can be a bit subjective. It also depends on what you're going for. If you want your MC to have this super mysterious background you probably don't want to drop a giant info dump about them in the first chapter. It can feel a bit unnatural if your character is all, "Hi, Random Stranger. My name is Jane. Let me tell you my entire backstory."
You can still put in details without giving everything away. In Dollface it is shown from the prologue that my main character has killed people. What I don't tell people right off the bat is when/how/why she got out of that life. You could also try alluding to things through the way your character reacts to different scenarios.
Of course, this varies from character to character and what works for me might not work for you.