In principle, the smallest change possible is a single nucleotide substitution: the change of one base of DNA into another. Or, similarly, the insertion or deletion of a single base.
The reason I'm curious about this is because it would affect the speed of evolution. The more genetic mutations needed the slower evolution would proceed.
It is possible to measure the rate of mutation, and to show that this accounts for the amount of evolution that's happened in the available time.
But you're talking about significant and useful mutations. That's harder to quantify.
Our ability to read genomes is fairly recent. People are still trying to figure out what were the
significant mutations that make, for example, humans different from chimps.
If you're still posting on these forums in twenty years, bump this thread.