I can't help you too much, but try standing over the frames. The top tube should be at least three inches below your crotch (mine is more like 4 or 5 I think). When seated on the saddle, you should be able to reach to the handlebar and maintain an upright position but it shouldn't be as if the handlebar is right in front of you (this ensures the top tube isn't too long or short) - imagine what it'd be like on the trails: would you feel comfortable or are you to bent over?

Finally, make sure that when you're seated your feet can rest on the pedals in a manner allowing your knees to be slightly bent when the pedal is in the lowest position.