You need to do a gauge swatch in the yarn you want to use and come as close as you can to the gauge in the pattern. Then with measurements of both you and an article of clothing that you like the fit you can better judge what size.
Also.. that is a cute top. I've heard the name, but never saw it or heard of the patterns.
Oh! I just checked Ravelry.. are you on there yet? Anyway there are 177 Rusted Roots being made. I'll take a look at them and see what they say.