My red tail boa isn't his full adult size after a few years now.?

Genetic disposition, proper temperatures, and feeding frequency and prey type are all contributing factors into the boas size and growth. The first 2-4 years is the most critical in growth. You have had him on a very irregular schedule and then didn't eat for a year. Also 5 ft is an adult size for boa constrictors. Adult size is quite variable though the subspecies, the smaller one being 4-6 ft adults and the larger 7-10 ft.