Of course, the less of that substance you have, the less radioactive it is, and the slower it decays.
This is the first time I've heard that the rate of decay depends on the mass of the isotope in question - rate of decay is a random internal atomic process. A single radioactive atom will breakdown at random (all by itself). It's the combination of all that random breakdown that brings about the phenomenon of half-life. The smaller the sample, the more random (faster OR slower) the breakdown becomes. And as for size being a problem - proteins are fairly large molecules. The problem will be not the size of the individual atom but whether or not the villain has the capacity to get the isotope 1)into an injectable solution, and 2) broken down finer than a protein or even a blood cell.
And the longer the half-life, the less radioactive the isotope is. I recall reading that iron has a half life in the billions of years.
