To get more room for code, you can create a number of extra “PROG” banks as declared in project.json. Once you've created those banks, you'll need to mark your functions and variables to indicate when they live in a non-default bank.

Functions and const variables have different markers in the form of Compiler Pragmas that should be wrapped around their definitions. See the CC65 documentation for full details, but the important parts will be explained here as well.