In practice, IME, a good rule of thumb is that if you're programmatically generating any of the rules then the grammar is probably too big for a packrat parser

In practice, IME, a good rule of thumb is that if you're programmatically generating any of the rules then the grammar is probably too big for a packrat parser to be fast enough on realistic inputs. If not, and if the entire parser is written by a single person in a time-frame that would make sense for a parser, then things are probably gonna be ok in all but the worst implementations. YMMV