Hi, You need to follow below steps.
In client side js, options can be passed from respective webscript. For documentlist.js follow below steps
1) Add page size value in property file. documentlist-v2.get.properties located \share\WEB-INF\classes\alfresco\site-webscripts\org\alfresco\components\documentlibrary. Add like "page.size=20"
2)Add this page size as documentList options in js file - documentlist-v2.get.js
pageSize : msg.get("page.size")
3) Refresh your script . This value will override hardcoded pageSize value in client side js file.
Need to add all changes in extension.