First off I think I should sort out a misconception about Huygens Principle. You can apply this principle efficiently if you have a slit, which is equal or smaller than the wavelength you are considering. If on the other hand the slit is substantially larger than the wave length, you should consider multiple Huygens sources.
Take a look at this animation
from wikipedia.
Diffraction of a plane wave when the slit width equals the wavelength
As you can read in the description of the animation, the wavelength of the waves are equal to the width of the slit and you see a nice demonstration of Huygens Principle.
However as the slit gets wider and wider the Huygens Principle breaks down and you have to consider multiple Huygens sources as it is illustrated in this diagram from wikipedia:
You can immediately see that when you make the slit larger, the diffraction effect becomes less pronounced.
Your second question is explained in this answer, I suggest you should take a look at it.