A Question to Brits re. Living in Spain & EU Visa! (Barcelona)

Hi there! I am a British Citizen and wonder what the rules are regarding staying in Spain. Being that its part of the EU can anyone from an EU country stay there for as long as they like? I am a Brit currently living in California with a British passport and will shortly be making a trip to Barcelona and have a little idea in my head that maybe I would like to stay. Would it be possible? If not what is the maximum length of stay as a tourist? Do I have to show a return ticket?