Sounds like you got O.K. advise...

I am 5'7" with a 30" inseam. I find a "small" of 14-16" gives me much better control.

My commuter is an old 20" Raleigh hybrid with the seat slammed all the way down against the top bar. Once I get going down the road it is fine and I like the extra height when around traffic but it is uncomfortable at slow speeds passing between cars in parking lots etc.

I have a 1989 18" Rockhopper that barely gives an inch at the crotch that I use for trails and when I'm stopped sitting on the seat I can only touch the toes and front ball of my foot to the ground. On the 89 the crossbar is straight across like on a road bike. I really wish it was a 16" or had a sloping bar.

I also have a 14.4" M30 that has a slightly sloping bar that I could probably use a larger size of based on the "measurement" schema but I find when riding it I have much better control and maneuverability.

If you are a campus rider you will find yourself working your way around parked cars, uneven sidewalks, curbs and the occasional trip through the grass. I think the MB advice was sound and the size O.K. as well. You might want to eventually get some street (hybrid) tires, they do make a world of difference in the handling and can still deal with the occasional trail. Stay away from fancy crap like shocks and just stick to an old basic rigid frame, less to go wrong and cheaper to replace if you get jacked.

Other than that just go off of what feels comfortable. You may want to try a few bikes in the 16" range but every style of bike will vary so you need to try all the sizes (XS, S and M) in each one to get the feel for how each size change affects the fit and handling.