Why are the Planck length and time considered the smallest units of length and time?

[Strinka]

I don't really see where that comes from. They're really small, but so are the Planck mass and charge and they're not the smallest possible units of mass and charge. Rather obviously so, since the mass and charge of an electron is less than the Planck mass and charge.

So, why those two?

[Exapno Mapcase]

The planck units are useful because they are derived from basic constants.

From Wiki (http://en.wikipedia.org/wiki/Planck_units)
In physics, Planck units are physical units of measurement defined exclusively in terms of the five universal physical constants shown in the table below in such a manner that all of these physical constants take on the numerical value of one when expressed in terms of these units. Planck units elegantly simplify particular algebraic expressions appearing in physical law. Originally proposed by Max Planck, these units are also known as natural units because the origin of their definition comes only from properties of nature and not from any human construct. Planck units are only one system of natural units among other systems, but might be considered unique in that these units are not based on properties of any prototype, object, or particle (that could be thought of as arbitrarily chosen) but are based only on properties of free space. ... They eliminate anthropocentric arbitrariness from the system of units...

Their size, therefore, doesn't have to correspond to pieces of matter. Instead, they are often limts:
In fact, 1 Planck unit often represents the largest or smallest value of a physical quantity that makes sense given the current understanding of physical theory. For instance:

A Planck velocity of 1 equals the speed of light in a vacuum, a maximum;
At lengths and times of less than about 1 Planck unit, quantum theory as presently understood no longer applies;
At a Planck temperature of 1, the four fundamental forces unify and all symmetries broken since the start of the Big Bang are restored.

[MikeS]

Essentially, it's because General Relativity describes the structure of space and time. We don't yet have a quantum theory of GR, but we can still estimate the scales on which we expect it to become relevant by combining the constants associated with GR (G and c) with the constant governing quantum mechanics, h. The results are, of course, the Planck length and the Planck time.

Note, however, that it's not entirely accurate to say that "the Planck length is the smallest possible length". Rather, one should say that we have no reason to believe (or disbelieve) that our current models of space and time are applicable below this scale. It's entirely possible that if and when we come up with a quantum theory of gravity, viewing spacetime as an infinitely divisible continuum will turn out to be valid; but it's also possible that it won't.

Oh, and as far as the Planck mass is concerned, you can view it as the mass of a black hole whose radius is the Planck length; so it's related to the "minimum possible length" in that way, in the sense that we don't know how smaller black holes would behave.

[Chronos]

It's also worth pointing out that most of the Planck scales are only estimates, based on dimensional analysis. Nobody would be particularly surprised if the fundamental quantum of spacetime (if there is such a thing) turned out to be 2pi times the Planck length, or 1/137 of it, or half of it, or the like.

The Planck action and the Planck speed are both fundamental limits, however, and are both known precisely. You can't have an action (or change of angular momentum, which has the same units) smaller than the Planck action, and you can't have a speed greater than the Planck speed. They're better known as h and c.