The thing that is too large is your P-code. Breaking out some of the functionality into separate procedures or functions is the obvious solution. But if you are unwilling to do that, then try defining views that can be used to simplify the most complex SQL statements. If you are only just over the 64K limit, then using shorter variable names might just do it.

I have a package which contains only one procedure of more than 1200 lines of code. When I compile I get an error: PL-00123 Program too large

I can't split that procedure. Any suggestion for me?