Where are you located?
You have two options. You can either go through your local Surly dealer and have them measure you up. You may get a sales rep. who is knowledgeable about Surly bikes and fit, or you may get a 22 year old who doesn't know the difference between a top tube and a downtube.
The other option is to visit a physical therapist who specializes in bike fit. I live in Seattle and there happens to be a bike fit specialist who sized me up for a custom frame. He essentially matched me up to a standard geometry bike and then tweaked the headtube and TT length. While you aren't looking for a custom frame, a bike fit specialist would at least be able to point you to the appropriate standard geometry fram. Note: this is a costly route.
I wouldn't trust anything anyone on this thread tells you about what bike size is right for you. You can't really fit a bike over the internet, particularly based on comments of people who may or may not have any experience in the area.
I'd try to find a shops that have a few Surlys built up already and see how they fit you. This is challenging since most bike shops I've visited don't build up framesets.