how to make big menu in drupal 7
You can do this by using any of below modules :


Megamenu



  Mega menus are a type of drop-down, which seeks to overcome some of
  the usability drawbacks to traditional style drop-downs.



Bigmenu



  THIS MODULE attempts to replace the all-in-one global tree view with a
  similar interface, but sub-trees are "collapsed" below a certain
  level, and can be expanded as needed.



TB Mega Menu



  TB Mega Menu allows you to create a mega menu with an innovative
  back-end user interface and synchronized with Drupal core menu.



OM Maximenu



  That is now possible by using OM Maximenu which can create menus with
  all other module blocks you want as attachments, modules like views,
  slideshow, menu, user, nice menus, quicktabs, and custom blocks with
  tables, lists, images, videos, etc.



Superfish



  Superfish integrates jQuery Superfish plugin with your Drupal menus.



Menu Views



  Allows menu items to render views instead of links. This is useful for
  creating "mega-menus".



MuchoMenu



  This module allows users to create mega menus that have mini panels as
  their flyout content. The benefit is that ANY type of content or UI
  can be added to the menu flyouts.