The wattage rating on a speaker means little. 1 watt of distortion can blow a speaker much faster than 100 clean watts.
You might want to look at more efficient speakers instead of their size or age. This is why vintage Klipsch, Cervin Vega or Realistic Mach 1's all with horn drivers can be desirable. They can be rated around 94db to 100db at 1 watt, 1 meter at 8 ohms. 40 watts can drive many speakers to "reasonable" volumes. Personally I would avoid anything under 90db at 8ohms.
You might want to look at more efficient speakers instead of their size or age. This is why vintage Klipsch, Cervin Vega or Realistic Mach 1's all with horn drivers can be desirable. They can be rated around 94db to 100db at 1 watt, 1 meter at 8 ohms. 40 watts can drive many speakers to "reasonable" volumes. Personally I would avoid anything under 90db at 8ohms.