There's a deceptive amount of custom coding involved in this. At the most obvious level your would need to track down the code for each of the listings and find out the name of the Admin Switch that defines the items per page for that listing, intercept where that is applied to a variable for use in the SQL database extraction and insert the value you have gotten from the visitor.
But of course, that's just the start. You also have to get the value from the visitor via a form of some sort and save it into both a session variable (for immediate use) and most likely a cookie for future use.
Then when a visitor returns, you would need to read in the cookie and set the session variable value automatically.