Question

When would maximum diffraction occur for a single slit?

Answer 1

This happens when the slit width is as small as possible.

The above is not quite true, and it has a few limitations as well.

Limitations

The narrower the slit, the less light there is to diffract, you will reach a practical limit, unless you have an enormous light source at your disposal (but even then).

If your slit width is in the neighbourhood of the wavelengths you are studying, or even below, some or all waves won't make it through the slit. With light this is hardly ever a problem, but with other electromagnetic waves it can be.

This is one reason why you can look inside your microwave, and still be safe from waves leaking out -- the holes in the grid are small enough to be a barrier for the microwaves, but large enough for light.