There are 8 bytes of ICMP header and 20 bytes of IPv4 or 40 bytes of IPv6 header for an Echo Request, so with -s of 8000, each ICMPv6 packet is actually 8048=40+8+8000 bytes, and for ICMP it’s typically 8028=20+8+8000 bytes.
For the common 1500 bytes MTU, use ping6 <addr> -s 1452. See also Path MTU Discovery.