I think all snakes require the opportunity to climb if the wish to for enrichment and quality of life. Same as they require a hide should they wish to use it at any given time. So I think at least 4, maybe push it down to 3 feet of height for a Retic at a minimum. That's just my opinion from what I've seen of their behaviour in the wild. But if you have a lot of animals space might be the most important consideration.
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