Yes, the venue is pretty big, but don’t worry – you won’t get lost (unless you decide to get lost in music, but that’s a different story).
Your sector is indicated on your ticket. Simply find it on the venue map, check the route and go to the corresponding entrance. There will also be navigation banners and signs at Ušće. And you can always ask any venue staff member to point you in the right direction. GA tickets purchased on gigstix.com (http://gigstix.com/) allow entry through any entrance, regardless of the sector designation written at the entrance.
But please note that you can only enter the venue once. That means that you will not be able to return to the venue once you have left it.