While there were many sites attempting to create models with avatars, I did not see any doing something quite like this in terms of image matching or multi-models. I stayed away from many of the design inclinations of the avatar-based sites since the functionality is completely different.
To validate my conclusions and get feedback, I spoke to several colleagues, friends, and a product manager to discuss my wireframes and user flow. Their feedback was critical in designing a better UX and it went through several iterations. There are many aspects of this user flow that would need to be A/B tested to see how users interact with the tool and determine the best way of showing options and minimizing steps.
The overall placement of the CTA will most likely vary between companies and what they want to be emphasized. CTAs that are not primary, but aid in conversions should be placed above primary CTAs most often.
CTAs should be placed next to where users typically make selections (size sorting, in this case) as it is directly related to the user behavior and minimizes user's movement around the page.