I'm 5'5" and ride a 17", but I have about 4-5 inches of clearance. Although you will do mostly road riding, the rule of thumb is 4-5 inches on a mt bike, 1-2 inches on a road. In case you might start playing in the dirt with the bike I'd go with a smaller frame. I had an 18.5 inch for a month, and trust me smaller is better when it comes to frame size.
-Middi-zon
__________________
That's just my opinion, I could be wrong.
Giant Rainier
Giant OCR 3