Concrete a garage floor- does it need expansion joint?

5 x 5 is ok without an expansion joint but you must use reinforcement mesh (type A142) would be ok for a garage. Must be at least 150mm thick and founded on type one or at least a sub soil that ALL topsoil has been removed from. Good practice would put a foam, that comes in strips or rolls around the perimeter but only if you have a wall outside it! Is the floor to be a foundation for the walls as well ? 5m is ok to screed, you are best though, especially if you aren't used to it, to put a screed rail down the middle at the right height (make sure the mesh runs through it though). This rail you must pull out during the pour and replace with the fresh concrete, take care to get good compaction here. I will do it for you if you are local.