I have never been to Iznik... however I have been to many different octogar. And I have always been able to find a place to leave my luggage. It may be in a locker... if not, there might be proper 'left luggage' places... the ticket seller might allow you to leave your luggage behind their desks.... or they might even ask you to just leave your bag outside their office (yes no lock, nothing! But turkish people tend to be honest. Of course... everything will be left at your own risk)
So YES, you will definetely be able to find a place to store your luggage. However, it may or may not be a proper left luggage place. So just be carefuly to not leave anything of value inside your suitcae.