I started with the small size, reasoning that it was close in stitch count to my default vanilla sock, I was using an ever so slightly thicker yarn (Socks Yeah! is a 3-ply, most sock yarns are 4-ply) so I'd get a slightly bigger end project, and the pattern is lacy and lace opens up to be larger than plain knitting. Yeah, that didn't work. I turned the heel on the first sock and knit for about an inch, tried it on, and decided it was not going to fit comfortably so I had to frog and start again with the medium size recommend for my foot size.
Between having to knit the first sock almost twice and July being a short month for AYoT, for the first time the following month's pattern was released before I finished that month's. But I finished and that's the important thing.