Unfortunately I don't think it can currently be changed without overriding the Index
content class.
If I remember correctly, the number is in 3 places: the API controller (can be changed via extender), the javascript state class (can be monkey-patched) and the content class (cannot be changed without overriding the class).
So no, it cannot be done just by editing extend.php
(unless inline-ing a ton of javascript and PHP code)
I do hope we can consolidate that into a single constant in the future to make changing it easier.
Something else that's currently not supported is having the initial page have a different length from subsequent pages. I don't think the logic supports it at all.