Best practice for large source code projects

[dhable at gmail.com]

I'm developing a web system and based on some patterns I've seen
elsewhere, I made a single  file (model.py) to hold all of the
functions and classes that define the model porition of the
application. Hence the code in the controller looks like:

import model

def Display(req,id):
  # ....


It works and things make sense to me. Yet, I feel uneasy that my
model.py file is starting to approach 500 lines. What's the best
practice or community opinion on this? Do I keep everything in a single
file or do I start dividing things into separate files?

TIA