I remember reading the Scientific American book adapted from Charles and Ray Eames'
"Powers of Ten" films, and noticing that the pictures ran out at 10^-18 m (distances covered by quarks dancing inside a proton) on the small side, but went up to something like 10^25 m on the large side (a billion light years-- today we'd probably want to draw one picture more and include the results of the great redshift surveys, at what will probably be the largest order of distance magnitude ever directly accessible to observation, because of the finite age of the universe and the speed of light). Going by that, we'd be a few powers of ten closer to the small frontier than the large.
On the other hand, the position of the small frontier is open to revision and quibbling as well; the pictures run out as much because of the inadequacy of pictures in a quantum world as the inadequacy of human knowledge. A wavelength of 10^-18 m corresponds to an energy on the order of 2 GeV, which is just the rest mass of a couple of protons. Modern particle accelerators can accelerate particles up into the TeV range and often study interactions with center-of-mass energies of hundreds of GeV, so you could argue that 10^-20 or 10^-21 m would be more accurate today.
I should also say that no nonzero
size for electrons or quarks has ever been detected: if they're something other than point particles, it's yet to be determined.
And, of course, people routinely
theorize about distance scales insanely smaller than 10^-21 m: string theorists and quantum-gravity people suspect that the whole concept of "length" breaks down around 10^-35 m, and like to speculate about what happens at that point. But, then again, quantum cosmologists like to think about scales way bigger than the observable universe, as well.
