A quick and easy fix for this is to make use of a plugin called Force Regenerate Thumbnails (to which I don’t have any affiliation to)
Unlike a plugin like Regenerating Thumbnails, Force Regenerate Thumbnails creates all your new custom sizes and delete all redundant/orphaned sizes.
Force Regenerate Thumbnails allows you to delete all old images size and REALLY regenerate the thumbnails for your image attachments.