default or selected kernelversion does not have config value CONFIG_IP6_NF_MATCH_LENGTH.
Result is shown for kernelversion 5.4.123

Packet Length match support

modulename: ip6t_length.ko

configname: CONFIG_IP6_NF_MATCH_LENGTH

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>Network packet filtering (replaces ipchains)
└─>IPv6: Netfilter Configuration (EXPERIMENTAL)
└─>Packet Length match support
In linux kernel since version 2.6.12  
This option allows you to match the length of a packet against a
specific value or range of values.

To compile it as a module, choose M here. If unsure, say N.

source code:
net/ipv6/netfilter/ip6t_length.c