But you shouldn't optimize your purchase for "beginner" skill level unless you plan to suck for several years. If you really want a beginner board, get a cheap throw-away on craigslist and use it for a year.
BTW, anything with an "N" in the length means narrow. So try to find things that are not that.
Also, check this calculator:
Snowboard Sizing Guide, Size Calculator
It puts you around 153-155. If you stray from your target range, going smaller makes it a lot easier than going longer. Longer only helps you stay stable when going fast, and you'll have to be going fairly fast for a beginner or intermediate to reach the limits of a board that is a few cm too small. Smaller makes turning easier. At your height/weight/shoe size, you are going to find that you need a shortish board but have your feet much wider apart on the board, using the outside screw holes. You'll also find that anything with an 'N' probably won't work. Having your feet so far apart does buy you a little more clearance because the board is wider near the ends. It shouldn't be a real problem but you will want to test fit everything before you buy if possible, or really understand all of the measurements. Most people within the averages can kind of just pick something roughly close and not worry about it.
Also I don't think the guys at the shop were too far off when they said that the boards are made the same for men and women. It seems like women's boards are just smaller lengths and feminine graphics. Think about the difference between a shorter guy and an average woman. The weight is in slightly different places but really, what are you going to do differently to a piece of wood/fiberglass around your feet differently?
I know for my board (Lib Tech TRS), they literally just put girly graphics on the topsheet for the shorter sizes/narrow width and stuck them in the girls section. My girlfriend is 5'10" and I've learned that the industry does not seem to cater to females of that size.
Most important thing though, BUY YOUR BOOTS FIRST.