For models of the SE5a, Fokker D7, Spad XIII and the like 35CC engines are kind of the baseline. You can go as light as a 25CC class engine on planes planes like a Camel, Pup or Triplane but models of the larger liquid cooled fighters usually need the additional horsepower. I've seen planes with lots of detailing that flew on 50CC class engines. Given that I don't know anything about your building or flying habits I would recommend being conservative and shoot for a 40 CC engine.