It's just as easy or hard to map out a VPN's egress subnets on v6 than it is on v4.

I assume for aggressive blocking the only prefix size will change. What is a /32 for IPv4 might become a /64 or smaller for IPv6.

Shouldn't blocking v6 also be based on /32 if you want the attacker's cost to be the same?

Well in the IPv4 world /32 would be a single IP or what a residential connection usually gets. With IPv6 you have 128 bits for the whole IP and usually a residential connection get get between a /64 to /48 prefix. Going above a /64 might hit other unrelated customers. Going to a /128 prefix would only block a single IP but since we started doing privacy extensions your computer will have multiple IPv6 addresses with a short lifetime which means that the user will be able to connect again soon after you block them. There are 18,446,744,073,709,551,616 IPs in a single /64 prefix so it would be useless to block every single one of them.

A more reasonable approach might be to block a /64 first, monitor if you get more blocks within the /56 block that contains the /64 and maybe block that.

Larger. A /56 and get multiple hits from nearby /56s and you block the /48.