The angular position of the minima in single-slit diffraction is given by
$$d\sin{\theta_n}=n\lambda$$
so a wider slit width $d$ will cause the diffraction peaks to move closer together.
In double-slit diffraction, the width of the slit is typically taken to be negligible compared to the separation $D$ between the slits. The angular position of the maxima in double-slit interference is given by
$$D\sin{\theta_n}=n\lambda$$
so a wider separation means closer interference peaks. In addition, there is a 180-degree phase shift in the angular position of the interference pattern as compared to single-slit diffraction.
In practice, the slits in a double-slit interference also have a finite width, which means that single-slit diffraction occurs from each of them. Because $D$ is by necessity larger than $d$, the double-slit interference pattern will have many more peaks/radian than the single-slit diffraction, so the single-slit contributions are typically thought of as an "envelope" which gradually modifies the intensity of the double-slit pattern.
Interference effects can be observed in any particle that considers the wall in which the slits are cut to be opaque. So a neutrino beam, which doesn't interact with the wall very much at all, would exhibit essentially no interference pattern.
I am sure that your laser pointer is fine for the job.
The cross is due to the fact that your hole is square.
Try making a six sided hole and see what you get - 3 lines inclined at $60^\circ$ to one another?
It is due to the diffraction of light and called a diffraction spike or starburst effect in photograph as shown below.
If you do not want the starburst then use a circular hole.
Best Answer
One way of understanding this which has always had intuitive appeal to me is the so-called Huygens Principle which basically states that every point on a wavefront can be considered a point source for a new spherical wave, and that the evolution of the wavefront can be determined by superposing all of these spherical waves at later times. The Wikipedia article that I linked to has some really nice pictures of this.
Diffraction effects can then be explained using this principle. Imagine, for example, that you shine light through an extremely small slit, say a slit about the size of the wavelength itself, then when plane waves pass through this slit, the part of the wave that goes through the slit acts as a point source and generates a spherical wave, so the light diffracts.
If the slit is larger, however, then the part of the wavefronts that pass through the slit act as multiple little points sources for spherical waves, and these spherical waves interfere with each other to give an interference pattern. In this way, the diffraction pattern is very much like multiple slit interference, except instead of multiple slits, the wave front itself splits into a bunch of adjacent point sources that interfere with each other.