The book began with a human male accidentally being sold as a slave to a clanship of aliens. The aliens were much bigger and stronger than the human so ended up protecting him from their own kind. The human eventually was adopted by the clan. I believe it developed into a series and I read it more than 25 years ago. Thanks in advance.
Could it be Foreigner by C. J. Cherryh? It seems to tick quite a lot of the boxes with aliens being substantially larger than humans, using slavery and operating in clans.