What will a very big observer compared to humans observe if he uses humans on a double slit experiment? Suppose there is a huge world in our universe and their laws of physics is the same as ours. Humans are like atoms compared to that world. In that world a double slit experiment is being performed by an observer where instead of electrons humans are fired through a  narrow slit.
So when humans are fired will the observer observe quantum effects? Will the observer observe interference patterns or human diffraction? And what will the human observe?
 A: To observe interference in a double slit experiment, what is important is that the wave remain coherent. What that means is, the wave, in an approximate momentum eigenstate, must keep all its components oscillating in phase in order to remain a wave.
We all know that any experiment you do to determine "which slit" the wave goes through will destroy this coherence and no intereference pattern will be observed.
So to do this experiment, you need to first prepare a human in a momentum eigenstate. This is practically impossible, because you need to make sure all 10^28 or so particles of the human are oscillating in phase, and you also need to protect them all from interacting with particles in the environment (which would become entangled with it and carry away coherences).
But if you could somehow prepare such a state, and then isolate it from air particles, radio waves, cosmic rays, cosmic microwave background, and measure electromagnetic radiation from the human (e.g. IR radiation from heat), and all beta decay products (like neutrinos, etc.), then in principle you could observe a human interference pattern.
A: Take a billiard ball, make two slits the width of the ball +1 cm, 1 cm apart, and start throwing the ball at the slts and marking the hits on a screen a given distance away. What will happen? most of the time the ball will hit the plane where the slits are made, and bounce back.Some times it will go through without touching the slit walls, and sometimes it will go though grazing the slit walls which will give the trajectory an angle. If you plot the hits on the screen you will get two lines, the images of the slits, plus a tail from  the  balls that grazed the slit walls. See this.
The double slit experiment you envisage with humans would be the same , in addition with a lot of blood on the  wall where the slits are .
The following  is the mistake.

Humans are like atoms compared to that world

Atoms are quantum mechanical entities, humans are statistical ensembles of quantum mechanical entities, and these ensembles obey classical physics, not quantum mechanics laws.(For sizes larger than nanometers, quantum mechanics is irrelevant, except in special cases, as supeconductivity) Your imaginary being would still have to go down to atomic distances to see quantum mechanical effects.
A: Quantum effects do not "scale" the way you think they do. To enlarge slightly upon Anna V's answer, the fundamental scale of quantum effects is set by the numerical value of Planck's constant. This number is very very tiny, which means that quantum effects only kick in and get important for very very small objects.
To make quantum effects important on the scale of humans would require Planck's constant to be correspondingly large. Experiments (and our everyday experience!) show it is not.
To make quantum effects go away completely, Plancks' constant would have to be exactly zero.
