You need to put a file called, for example, localhost inside /etc/modprobe.d/ directory.
Inside this file, add these lines (value is an example):
options nf_conntrack hashsize=333333
And now, the more quick solution is a restart, the other option is to try a reload the nf_conntrack kernel module, wich is a bit difficult because it's linked with others running modules.
Check the result with: