I don't know exactly what you mean but the size of a group affects its defense because there is a larger group. Therefore, more people would be protecting it, adding more to its defense.
The answer is Observation. That's the first step.
Prokaryotes- lack a cell nucleus, unicellular, in the bacteria and archaea domain, lack mitochondria and chloroplast, microscopic
eukaryotes- contains a cell nucleus, multicellular, only in the eukarya domain, contains mitochondria and chloroplast, not microscopic