[n4r9]

I'm not sure I understand - why is the wavelength size relevant? In my understanding, the standard deviations of position and momentum are at least some constant multiple of Plank's constant.

[eru]

Sorry, I meant to say that for particles (or collections of particles) with a short de Broglie wavelength classical effects dominate quantum effects.

The typical scale at which quantum effects dominate for a system is roughly equivalent to the de Broglie wavelength of the relevant parts.

Does that make sense?