For many clothing products, Amazon displays a histogram of how previous purchasers have found the fit (too small,..,fits perfectly,..,too large) and a summary (51% of customers say this runs large).

shoefitr is so close.

Now let me scan my foot instead of "I mostly fit into size X", and you've got a great solution.

Google is starting to sell tango tablets which include 3d-scanning , for everybody ,for $512. So maybe we're not far from what you suggest.

Scan? A picture of my foot next to a ruler, please. Or some other standard-sized reference object, like an electrical outlet.

Maybe men's shoes are different, but in women's shoes, you want to know the shape of the arch. Trust me, it matters :)

A 2D scan, though probably not what the parent comment intended, implies a size via a fixed DPI. A 3D scan, similarly, can measure actual dimensions without requiring a reference object.