/etc/postfix/main.cf

message_size_limit = 20480000

The default when message_size_limit is not specified is 10240000 (10MB).

For complete parameter list of main.cf, see the postconf(5) man page.

$ man 5 postconf

message_size_limit (default: 10240000)
       The maximal size in bytes of a message, including envelope information.

Caution

Be careful when changing message_size_limit.
message_size_limit must not be larger than mailbox_size_limit.
If message_size_limit is larger than mailbox_size_limit、you can't receive any messages.

The default of mailbox_size_limit　is 51200000 (50MB).
Zero means no limit.

mailbox_size_limit = 0

mailbox_size_limit (default: 51200000)
       The  maximal  size  of any local(8) individual mailbox or maildir file, or zero (no limit).  In fact, this limits the size of any
       file that is written to upon local delivery, including files written by external commands  that  are  executed  by  the  local(8)
       delivery agent.

       This limit must not be smaller than the message size limit.