My shoulders, "center to center" appear to be roughly 18"... 45 cm... edge to edge more like 20", 51cm...

Of course those aren't high-precision measurements... me with a tape measure . But still, it does sound like the 52cm ones are probably the correct size.

The nashbar ones are $25, the nittos seem to be more than that normally... I only found one on ebay for less, and it's unlikely I'll get it (still a day left). Since I really don't know how much I'm going to like 'em I don't feel like spending much on 'em.