How to make sqlplus prompt longer than 50 chars?
Finally, I found an answer!

The trick is, that the 50 char limit is for the string before the variable substitution! The size of the substituted string isn't limited. In my case, shorting _connect_identifier to a temporal variable _ci was enough:

DEFINE _ci=&amp;_connect_identifier
SET SQLPROMPT "(long things)_user'@'_ci'(long things)"