Here is a link to a chart. You can easily determine your frame size by just measuring your elbow.
Elizabeth, if you have a small frame, you probably do want to get down to the 135. If you have a large frame, you need to reevaluate your goal. A medium framed woman of your height they recommend 133-147, so you are almost in that range.
I'd be 5'9" in the one inch heels that they say you should be wearing for your height measurement, and I measured my elbow and I have a large frame So they say my ideal range is 149-170, which more then my original goal of 148 which just seemed impossible to get to in any healthy way. This chart made me feel a lot better about the goal I eventually settled on which was 160, and even that has been a bit of a challenge for me.