Max message size limit, how to set?

In server config, at mail tab, I set maks message size to 100MB.

1st question here, does it effect existing mail account or newly createds only?

But if I;
postconf -d | grep message_size_limit
message_size_limit = 10240000

I tried changing value to 1 MB but it is still same,

postconf -d | grep message_size_limit
message_size_limit = 10240000

But in fact value is getting change in main.cf.