The physical size of the layout is not the factor you need to consider when selecting a transformer.
You could use that transformer on a large layout if you provided enough power feeds to the distant track sections and the only thing that transformer was powering was a single train with few lights and a modest locomotive drawing less than 2 amps at peak.
Remember, the size of the transformer is affected by the power draw. Even a 4-by-8 layout can overload a transformer if the equipment is drawing too much energy.