I want to pass the number of pages in a select box (10,15,25,50) and when I press some button or just doing the action (observer_field?) it will change the pagination, how can i do it ?
Make the select box submit params[:per_page] (name="per_page") onChange. You'll also want it to go back to the first page (params[:page] = 1) or else you could request an invalid page (e.g., there is no page 7 at 50 items per page when you only have 100 items, even if you were on page 7 when viewing 10 items per page). Pass per_page and page to the paginate call.
-Kyle