erc-split-line-length

erc-split-line-length is a variable defined in `erc-backend.el'.
Its value is

440

Documentation:
The maximum length of a single message.
If a message exceeds this size, it is broken into multiple ones.

IRC allows for lines up to 512 bytes. Two of them are CR LF.
And a typical message looks like this:

:nicky!uhuser@host212223.dialin.fnordisp.net PRIVMSG #lazybastards :Hello!

You can limit here the maximum length of the "Hello!" part.
Good luck.

You can customize this variable.