:D. Tl;dr: it's actually near minimal to dual stack and do BGP between 2 sites of redundant services with advertisement failover to the internet.
/48 is the smallest accepted IPv6 advertisement and /24 the smallest allowed IPv4 advertisement (on the net at least). IPv6 assignments are done on the nibble (/48, /44, /40, ...). Since I only have two sites I could have gotten away with a /23 and something like a /44 but then the RIR pointed out it'd probably be cleaner to manage to get a /40 and give each site a /44 at the same renewal cost. As to why, I've just always worked in or near the networking field and so it seemed cheaper/easier to run it all myself in the long run.
Through another very long story, the Org ID + ASN + IPv4 space + IPv6 space ended up costing something like ~$1k the first year and then ~$500 in yearly renewals. I'm not legally able to resell these for the massive market rate prices of /24s because of them being leased for $0 specifically to dual stack my core hosted services like DNS with my own public IP, but that's fine as I got them to use :).
The largest assignment I've ever had the pleasure of getting and deploying was a /32 (... of IPv6!) for an extremely large healthcare system. Certainly no great record smasher, but I thought it was cool to work on an IPv6 subnet so big it could have been an IPv4 route nonetheless! That one was still also very sparse of course, but similar kinds of reasoning ended up getting to that being much easier/better to work with than a dense assignment.
I also have a small MAC address range (a classical "OUI" sized range is 24 bits of addresses, I have 12 bits of addresses) for a small run of some hardware I was making for a particular customer use case. With the leftovers I put my laptop on it and get the occasional "how the hell?" when they see Wireshark decode my laptop MAC as "Dan_Smith_xx_xx" while packet capturing to troubleshoot.