You're right. Black holes can theoretically be any size, so long as matter is compressed to a density sufficient that the object's escape velocity exceeds the speed of light. If Earth were compressed to 1.76 cm (slightly smaller than a dime in fact) it would form a black hole of said size. The trouble is, the forces needed to induce such a cataclysmic implosion come from massive dying stars, which only yield black holes of several solar masses or greater. However, in the early universe, pressures were so high that slight inhomogeneities in the distribution of matter could have created regions of space with densities sufficient to create black holes. These could be any size, though black holes weighing less 10^11 kg would have evaporated by now due to Hawking radiation. For comparison, a stellar black hole of 5 solar masses (5x10^31 kg) would take 10^69 years to evaporate. Primordial black holes the mass of asteroids are also candidates for dark matter. Their microscopic size and lack of individual gravitational influence would render them virtually undetectable. A high speed black hole of this sort could pass through Earth with hardly any trace.