Hi everyone! I'm set on getting a bike and narrowed my choice down to the Trek 4300, however I'm unsure of which size to get. I'm 5'6" and about 130lbs. I rode an 18" yesterday and while I was able to do it I wasn't comfortable. Stepping down to a 16" things felt a lot more comfortable, but now I'm wondering if I should go down even further to a 13". I'm mostly going to use this bike in the city, but would like to leave it somewhat open to riding some dirt and rocks in the future. Any suggestions?