Sometimes the smaller modeling agencies are actually better for you than the big ones. Especially if you are petite. If a smaller agencies, boutique agency, that gets a decent amount of jobs for models accepts you, then go for it! Just because the agency isn't known world wide doesn't mean it isn't worth it. Sometimes it can be just the in you need. Also at a small modeling agency, if you are liked, sent out and start booking work, you will get more attention more often.
Commercial print modeling agencies and talent agencies accept models of all heights, colors, weight and size. Focus on the print market, the commercial market.
It is more fun to rock your personality in a Mc'Donalds commercial or hair company hair commercial, than to be all stiff and hung liek a hanger anyways. Focus on what a petite model can do...and don't limit yourself. Shoot with diversity, and create a compcard that shows your range. Imagine each picture being an ad or in a magazine, that is how you can decide which photos are best to submit. And keep submitting!