Most of my audiobooks are from Audible. I have backups downloaded to my PC (couldn't guess overall size off hand) and currently use iTunes with them, which is not really up to the task, but use Audible's app and just download what I want to listen to from the cloud. One of these days I'll start the long arduous task of making DRM-free copies of all 690 titles currently in my library (I keeps saying that and really should get started).
I've toyed with setting up a separate Calibre library with "blank" books as a way of sorting & tagging things and then generating a catalog so I have something to browse through, or using something like
All My Books but never have.