There are agencies that accept petite models. They best thing to do if you are petite is not to Target yourself to fashion agencies. The quickest way to get an agency is to stick to either commercial print or talent agencies. Even if the height requirement is still 5'7' at these agencies I would still submit, usually a print agency is more likely to accept you if you send quality photos and they like your look. Images mean everything. They are what speak for you. At some castings sometimes you don't even speak you just hand over your compcards and portfolio, so really it is your images, your personality and your look that needs to come through in your images.
And not to get down if you don't get an agent right away because even without one you can still get work as a model.
Why not mail your photo to a magazine yourself? Why not look into brands, stores, and launching brands that might need a model. This could involve going to a tradeshow, taking a trip around your town, going to a barnes and noble or borders to check out the magazines that might need models like Hair magazine, Craft and food and travel magazines, lifestyle and bridal magazines. Remember modeling isn't ONLY fashion. Usually if you can build a portfolio and get a few tearsheets and agency will accept you.
I never think an agent is enough, I think no matter what you need to be a hustler, hard worker and always looking out for your own goals.