Ugh...I just had a long post and lost it. I'll summarize:
As an online buyer, I don't want to have to speak to anybody, at any time, during my purchase. That's one of the main reasons why I buy online.
CG, I know you probably don't want to do this, but...as a customer, I would prefer to be able to pick a size upon purchase. So, I would want to see a sizing guide, maybe as the last photo.
In the listing, you tell me what parts you need measured, I do so, then compare to the sizing chart, which would correlate with a size in the drop down options.
It may not be as exact as emailing/convoing back and forth but you could offer that as a "special treat" to those that want exactomundo.