I would definitely get measured.
I recently shopped for a road bike and the salesman visually sized me up for a men's frame due to my long torso. When we did the fitting my measurements ended up in the feminine frame range. I went feminine and I love it.

When I bought my mountain bike 18 years ago I went with a man's bike because back then all female bikes had deep slanted top tubes and were heavier. I chose my Rock Hopper because the top tube was shorter than the others I had short-listed. I ditched the RH for a hand-me-down Trek from one of my growing sons a couple of years ago. The dramatic changes in bike geometry from '91 to '04 made a huge difference. The worn out RH hangs from the garage ceiling these days. :-( But I love the front suspension on the Trek:-)

But a mountain bike is a different animal than a road bike. The more horizontal position is an adjustment for a new road cyclist (or returning riders like me) and a proper fit is critical. Get a properly fitted frame, it will enhance your riding experience in a multitude for ways.

Best wishes on your new sport!