NOTE: The following information has been superseded by itself, as scientific knowledge is by definition subject to constant revision. Thus anything that is said henceforth may be regarded as an illustration of quantum foam, in that the breakdown in structure it implies is embedded in the very structure of what is being said at the moment. This inherent self-referentiality and paradoxicality thereby temporarily converts qubikuity into a postmodern blog, for better or worse.
Someone mentioned quantum foam the other day as a pleasant-sounding thing, and I agreed that if you ordered a cosmic cappucino, you would definitely want it with lots of quantum foam.
Quantum foam refers to the miniscule scale where physical reality breaks down from the level of solid objects into complete immateriality. This is said to be at the incredibly tiny Planck length, or 10(-37) cm. The smallest bit of matter we know of is the quark. Now, I have been googling around to find out what a quark looks like. My guess is that it is not round like a tiny billiard ball. Anything that is on the verge of being dissolved into quantum foam is unlikely to be so regularly shaped. Furthermore, they are unlikely to be hard like billiard balls as well. My guess is that they have a spongy surface, rather like green cheese as in the case of the surface of the moon.
Of the six types of quarks (up, down, top, bottom, charm, and strange), the smallest is the up quark. The up quark is also known as a first-generation quark. By comparison, the bottom quark is quite large, but then it pops in and out of existence for only a micro-instant. Evidently, on that level, size doesn't really "matter."
For your edification I have found a picture of a bottom quark, again by googling. This is as accurate as present-day science can render it.