Protocol for requesting private data?

Sorry if this is the wrong place to ask...

I'm looking for details on how to use SSI to request access to 'private'
data from a controller. Specifically, I'm thinking about nuggets of data
that I don't want to put in plain text in a DID document.

My example is shoe size. An online shoe shop wants to know my shoe size to
taylor my browsing experience.

Are there specific / concrete proposals on how to negotiate this data?


Many thanks,
Dan.