For anyone wondering given that since 2014 no one has answered the specific question, it's rather simple and instructions were almost complete to change the values of the grid without having to modify the Core Files.

You need to create a main.py file inside a Folder named controllers on your Custom Module, it should contain this to replace at your liking the Products per Page and per Row, make sure PPR is a multiple of PPG:

• from odoo.addons.website_sale.controllers import main
  # path of the main.py that has the original values
  
  main.PPG = 21 # Products per Page
  main.PPR = 3 # Products per Row

Inside that very same controllers folder make a __init__.py file with the following inside to call the main.py file you just created :

• from . import main
  

And finally outside your Custom Module Folder create another __init__.py with the following code inside to call the contents of the controllers Folder you just created with the main.py File inside

• from . import controllers

Hope this helps someone.

Found it hardcoded at line 11 of /website_sale/controllers/main.py

PPR = 4  # Products Per Row

Probably want to make sure it's divisible by the amount set on line 10

PPG = 20 # Products Per Page

Still not sure how to override this without changing the core file...