The longer, the better?

To put it short: The longer, the better.
Most often a Beverage length is between 1/2 and 2 wavelengths.
How long that is depends on the frequency you want to listen to. For the Tropical 60 meter band (with a wavelength around, exactly, 60 meters) the length of a 2-wavelength Beverage will be 120 meters.
On the medium wave broadcast band 2 wavelengths will be around 600 meters.

To put it longer: the experts don't know, or don't agree.
Some technical minded argue that it is a waste of wire to make a Beverage any longer than 2 wavelengths.
Others more practical inclined argue that while 2 wavelengths may be technical true, 4, 6 and 8 wavelengths have proven great in real life.

Why long Beverages could be overrated

Loss, out of phase signals, and different fading times may not make the extra wire worth it.