It depends more on your inseam than your height from what I have read. That being said I'm 5'10" and prefer a 58cm bike. My friend whose 5'9" takes a 54cm, which i find horribly cramped (I never adjusted the seat post on his bike, nor the handlebars though). I'd suggest trying to ride it and seeing if your legs extend fully and whether or not you find it comfortable.
With that being said a 15" is the smallest frame that bike comes in and is the smallest size i've personally ever seen an adult bike so you could also consider looking for a child size bike instead (probably save some money that way too) if it is too big for you.
Here is trek's recommended sizes based on rider's height: http://www.trekbikes.com/faq/questio...?questionid=63
Best of luck and I hope the bike fits you perfectly
Edit: By the way, according to that sizing chart it says I should ride the 20" frame (there is no 19"), yet I still found the 22.5" frame much more comfortable. (I'm riding the 7.2fx , it lacks the front shocks the 7200 has and rides a bit less upright [0.5degrees less] )
Last edited by Matt905; 06-27-10 at 07:46 PM.