Can't you just ask her for her ring size? If she doesn't know it, maybe give her a cheap ring from Claire's or something in what you think might be her size and you can figure it out from there.
And also...I know rings are a lot, but I would suggest saving up so that you can get her a good one. Most jewelers offer insurance and even free resizing, so I think this is one of the times where it's best not to go cheap. If you think it would take too long to save up, you could give her a ring and promise that you'll get her the one she really wants when you can afford it. You could also keep an eye out for sales. And some places offer layaway and payment plans.
To be honest though, I don't think an engagement ring is a big deal- I love mine but honestly I would have been happy with anything. It's just a ring.
Here's the shop my ring came from:
http://www.etsy.com/shop/DesignMastersThey have a lot of different kinds, and several options that are under $1000.