I said I don't care what you do, I did not say I don't care what you think.
You and only you can decide whether it's too big or not.
Bike sizing varies greatly depending on frame geometry, but judging from your height, the bikes geometry and what the salesman said, I'm willing to bet it's too big for you. Changing stem length can help, but can only do so much... if that is the case then why not change it for something that fits from the outset?
At your height you should really be on a 54cm or 56cm.
Riding a bike that's too big isn't a great idea as you will be far too stretched out... which in turn leads to aches and pains... which in turn will most likely put you off cycling for life.