Step 1 is probably to see how many users on wiki have this preference set.

If we are going to actually deprecate the preference, step 2 would be some sort of user notice?

Parsoid Read Views currently does not support customized user media sizes, and neither does VE; there's an argument that editors should see the images at the size that most readers (logged out) will see them at while editing.

If it is not feasible to deprecate this functionality, Parsoid could implement a post processing pass to adjust media sizes, only for those users who have set a custom preference, but it will be relatively expensive.